Bathroom Wiring
First question has to be "What is a bathroom" when the BS7671 moved from 2001 edition to 2008 edition it dropped the section for showers in a bedroom and at the same time allowed the fitting of 13 amp sockets in a bathroom. In real terms there is no change as most bathrooms are not big enough to fit a socket 3 meters from the shower tray or bath tub. But shower cubical height is also important as if it reaches the ceiling or is over 2.25 meters high the area within the cubical becomes the bathroom not the area outside.

Section 701 of BS7671:2008 is 5 pages and even if I was allowed to reprint the pages it refers to other parts of the book and so is not really of any help. Also Part P page 8 makes it plain that it is notifiable work. Here is were regulations become grey as the definition of a bathroom varies. Also the paper work released for Part P has it seems not kept pace with BS7671:2008 and still requires supplementary bonding. Part F  building regulations require fan assisted ventilation of all moisture generating areas in a bathroom pipe min size is 100mm and intermittent extract rate is 15 l/sec and continuous extract rate is 8 l/sec for a WC at 15 minuet run on is also required. One new rule is all bathroom electrical items must now be protected with a RCD to regulation 415.1.1 which is no more that 30ma at 40ms because in the main the cable also needs protecting it would be normal to locate the RCD in or near the consumer unit.

Lights The bathroom fan is normally supplied from the lighting circuit as it is designed to come on with the lights it can go wrong so it is normal to fit an isolator so if it fails it can be switched off and still leave the lights operational. It seems to cause many problems fitting fans and because of the number of cables using a junction box is common. Although the diagram shows a fuse for all lights it does not show one for the fan. Because it has two line feeds to use an external fuse one would also need to fuse the bathroom lights and some manufactures therefore include the fuse in the fan. As with all electrical items one must read the manufactures instructions.

One warning when working on any lights is to carefully label the switch wire many a DIY person ends up with three blue cables and has not a clue which is the switch wire.

One other item often connected to the lighting circuit is the shaver point these have a maximum of 1 amp output and often include a lamp in with the socket many wire them only to work with the lights on so the transformer is not running when not in use but on most versions there is a built in switch and they only switch on when a plug is put in the socket. These units are for shavers only and the 17th Edition BS 7671:2008 lists all items that can be used in a bathroom and this does not include an electric tooth brush. I assume since they are rechargeable so don't need to be charged in a bathroom.

bath tub
There are now only 3 zones 0, 1, and 2 although still a 3 m horizontally limit from zone 1 before a socket can be fitted 701.512.3 was 2.5 m in old BS7671:2001 601-10.03 which in some bedrooms may mean a higher shower partition will be required. Zone 0 is the bit that could contain water i.e. inside bath tub or shower basin and as expected no electrical items are allowed here unless specially made for Zone 0 and manufactures instructions are followed it extends 0.1 meters high where no basin is used as with wet room. Zone 1 unless there is a fixed shower head higher is 2.25 m above finished floor level and area of bath or shower tray or 1.2 m radius from fixed shower head where there is no tray. Only (i) Whirlpool units (ii) Electric showers (iii) Shower pumps (iv) Equipment protected by SELV or PELV at a nominal voltage not exceeding 25 V a.c. rms or 60 V ripple-free d.c., the safety source being installed outside zones 0, 1 and 2 (v) Ventilation equipment (vi) Towel rails (vii) Water heating appliances (viii) Luminaires. I will point out they all must be designed to be used in Zone 1. Zone 2 extends 0.60 meters from Zone 1 unless no shower tray and then there is no Zone 2 and in Zone 2 you are allowed (i) switches and socket-outlets of SELV circuits, the safety source being installed outside zones 0. 1 and 2. and (ii) shaver supply units complying with BS EN 61558-2-5. Zone 3 only applies to sauna's now. Although I assume with a shower cubical over 2.25 m high it becomes a room in it's self it does say fixed partition and this is an interpretation matter. There are always grey areas and I advise always read the book yourself and don't rely on second hand information. There are in the BS7671:2008 nine drawing in all shower tray and bath being repeated one point that is made is where there is a window it includes the recess and it shows a recessed lamp also.

Shower cable
The question of cable sizes to electric showers is asked again and again one problem is with some showers physically connecting 16mm² cable can be near impossible so to just use the biggest is not always the best. There are 4 cables that could be used but mineral insulated cable is not really a DIY option so we have 3 the first is standard twin and earth because it has a limit of 70°C it needs to be relatively thick and where cable is de-rated going through insulated walls etc. then moving to a 90°C version of the same cable (normally white instead of grey) may assist without being too thick to handle there is also cable to BS 8436 designed to be used in partition walls which because of it's aluminium sheaf can be better cooled so can take even more current for same cable size. But most people would use normal twin and earth so I will stick to that here.

Size 100 <100mm thick 101 >100mm thick 102 touching 103 not touching C clipped direct A in conduit
6mm² 34 Amps = 7.8kW 27 Amps = 6.2kW 35 Amps = 8kW 23.5 Amps = 5.4kW 47 Amps = 10.8kW 32 Amps = 7.4kW
10mm² 45 Amps = 10.3kW 36 Amps = 8kW 47 Amps = 10.8kW 32 Amps = 7.4kW 64 Amps = 14.7kW 44 Amps = 10.1kW
16mm² 57 Amps = 13kW 46 Amps = 10.6kW 63 Amps = 14.5kW 42.5 Amps = 9.8kW 85 Amps = 19.5kW 57 Amps = 13kW

The table above shows the current rating for twin and earth from 6 to 16mm² in floors and ceilings (100 - 101) and in insulated walls (102 - 103) exposed and in conduit the conduit being exposed. As can be seen there is no such thing as 32 amp cable as some DIY outlets try to claim and if just shoved though the insulation without being clipped or other wise forced to be in contact with the cooling surfaces of a wall it may only carry half the clipped direct current. I have converted to kW as most showers are rated as kW. Miniature circuit breakers MCB's come in 32, 40, or 50 Amp in the main and rule of thumb would be a 32amp MCB for 6mm², 40amp for 10mm² and 50 for 16mm² and MCB's take time to trip and one does not spend that long in the shower so one can get away with slight overload so again rule of thumb 32amp for 7.5kW, 40amp for 9.5kW and 50amp for up to 11.5kW but as one looks at the bigger models one then has to ensure the power into the house is also enough.  

RCD requirement
With cables within walls under 50mm depth requiring RCD protection the practice of installing mini consumer units near to the bathroom will I would think die out. To use consumer units able to take RCBO's or with duel RCB's would I am sure in most cases be the way forward. In other areas of the house one may want to avoid Part P work but since the bathroom is Part P anyway that would be hardly an issue. RCD's however not only have the all important 30ma rating but also how much power can be sent through them with 63A, 80A, and 100A versions available. Showers would where combined with other items require the larger units. Where twin RCD consumer units are used the current using items in a bathroom and the bathroom lights should be on different RCD's as if something should go wrong and you get a shock the last thing one would want is to be plunged into darkness as well.

There is a set of condition that must be met if supplementary equipotential bonding is to be omitted this means all incoming services must have been already bonded as required in 411.3.1.2 and the disconnection times required in 411.3.2 are complied with i.e. cables are not too long. All final circuits are protected with an RCD to 30ma 40ms spec. To ensure these measures are met it will require minor works or installation certificates to be completed or at least all the tests required to issue those certificates to have been made because of the cost of the meters required around £700+ I do no expect many DIY'ers will be able to test as required.
But Bathroom electrical work is not really DIY anyway. The alternative is to supplementary bond
(i) metallic pipes supplying services and metallic waste pipes (e.g. water. gas) (ii) metallic central heating pipes and air conditioning systems (iii) accessible metallic structural parts of the building (metallic door architraves. window frames and similar parts are not considered to be extraneous-conductive-parts unless they are connected to metallic structural parts of the building). Supplementary equipotential bonding may be installed outside or inside rooms containing a bath or shower, preferably close to the point of entry of extraneous-conductive-parts into such rooms.  

I can't stress enough to read makers instructions. Items designed for use in bathrooms are of a much higher spec than run of the mill items. Test instruments and paper work required in order to comply with Part P mean it is not really DIY work.