Hi Huw,
This was intended as a short reply, it has got rather long - I hope it helps to frame your thoughts. I am no electrical or network engineer so you must take advice from the experts and ensure that the electrical installation is tested and certificated before use.
Installing the ducts, cable and wall boxes is very low cost at the construction stage, so best to buy wisely built in redundancy / spare capacity and do it only once - that is until you move house again!
Power: I agree with Mr Happy response, keep the hifi separate. Get the best quality electricity up to your power sockets. Your electrician is going to think that you a bit crazy and will advise you that this all seams a bit OTT/unnecessary, and that he/she has never had a problem of interference before....however, it is your ears that will know the difference and the costs are small compared to the value of the Naim system you will be installing (and then adding Naim Power-Line) . If you have space install a separate Consumer unit (dedicated for only hi-fi), with it's own tails to the electricity supply. You can install a dedicated circuit for the hi-fi using 2.5mm (normal mains cable) or you might choose to increase the cable size to 6mm or 10mm (I have no experience of running a bigger cable for sound quality), although I am aware of some installations that have followed this approach, to ensure the least degradation of the electricity supply to your hi-fi.
Number of sockets: 25% spares is always a good place to start, so much easier to install a spare 2-gang socket now - just in case.
Network: There are currently three flavours of cable, Cat 5, Cat 6 and Cat 7. Cat 7 is the latest and I believe is good for 1GBit/Sec (1000MBits/Sec). I agree with Simon-In-Suffolk, place them in ducts, install a spare, always leave some slack so you can re-terminate them. I try to run a single cable directly from my BT/Sky hub to the hi-fi / tv area and then install a '8-Port Gigabit Ethernet Unmanaged Switch' at the hi-fi end to enable all the kit to be connected. (remember the switch also needs a power socket!). I have had no problem with this arrangement, although you could run two cables from your BT/Sky hub to the Hi-fi/TV location and connect one directly to your music streamer and use one + switch for all the rest. This would use two ports on your Hub.
Speaker Cable: For the rears, it clearly Is personal taste, however I used QED Silver Anniversary XT Speaker Bi-Wire Cable, whilst outside the 'cheap as chips' category - you really only want to install them once!
What else:
Socket spacing and alignment: - it is worth thinking about this - all in a row needs a lot of wall space, all in a column means the cables from the top sockets hang down over the lower sockets, I went for an offset approach in 3 rows with lots of vertical space - rather then trying to get the sockets as close as possible.
Location of power & network sockets, Cat6 sockets - worth thinking about, temptation is to place them behind the hi-fi rack so that they are out of sight - makes isolating and unplugging a real challenge, find a location where there is easy access, without being coming too much of a feature of the room.
Sub placement - cable and power - does the AV system need one and where will it go, does it need power?
How many rear speakers 5.1 or 7.1 or more? What about ceiling speakers? - better to install cable now and leave them coiled up, even if never used. Keep good records and measurements!
What are you doing about cables to the TV, HDMI, Sat cables, TV/FM cables? If you are looking for a wall mounted TV, have you considered large ducting to ensure all the HDMI/power/etc. cables are concealed? Including future proofing when you change the TV and it needs more cables.
Use of Grid Modules for neat installation of cable termination on wall, I am sure you will have seen these modules, they are inserted into a 2-gang or 1-gang wall box and can be built up to cover every combination, Cat 6, Tel, Sat, TV/FM, Speakers, HDMI, etc. They have a brush option that allows you to have the cable coming through them - far neater than coming out between the carpet/skirting. - Have a search on the internet for the options, there are lots of suppliers.
I have just run cables to the rear speakers under the floor and installed wall boxes with Grid Modules, whilst only using 5.1, I installed bi-wire cable to each wall box so I have the option of 7.1 in the future. For the rears - each wall box has 4 - banana sockets. +/- for Rears and +/- for Surround. At the amp end I purchased the wires terminated with plugs to minimise the number of joins and these appear out of a wall box with the brush cover. As suggested by Simon-in-Suffolk, all my cables are in ducts to that they can be replaced in the future.
PS: If using the Grid Modules, make sure you install the deepest wall box you can - it makes terminating the cables much easier, 35mm or 47mm depth if possible. 25mm is a disaster waiting to happen.
PPS: buy a labeling machine, label and photograph everything (with a tape measure in the photos), it makes things so much easier in the future when you come to upgrade to make a change!
