What you should plan for is enough cable to run three separate networks. Here's what to do...
Distributed TV using aerial cable:
The aerial output signal from your Virgin box needs to go to a hub point - in your case it sounds like this will be under the stairs. There it will connect to an amplified splitter. This mains powered box will give you the output signals that you'll then wire to each bedroom.
The aerial amps come in three flavours. The most basic type cost £20~£30 for a 1 in: 4 out device, like this one from [urlhttp://www.argos.co.uk/static/Product/partNumber/5346161/Trail/searchtext%3EAERIAL.htm]Argos[/url]. The just multiply the signal, nothing more.
Next up in quality are the amps that allow 9 volt to pass through for remote control, like this
Philex one for less than £20. (Remote control of your cable box is a bit trickier than with Sky boxes which have the feature built-in ready)
At the top of the heap are the full signal distribution amps. These do the 9V pass, have more outputs, and can combine FM DAB Freeview (TV aerial) as well as the signal from the cable box, so no matter what you want to watch or listen to you'll have the correct signal at the socket. The cost is around £50~£80 for something such as this
Labgear box. If you are planning on staying in the house anylonger than 5 years it's worth the extra investment.
Video & Audio over CAT5e
This isn't really a network. It's simply using CAT5e (or CAT6 which is now becoming popular) as an alternative to AV or HDMI cables. The CAT cable on its own is dumb. It needs adapter boxes on each end called Baluns to convert an AV signal in to something that will travel down a CAT cable and then convert it back again at the other end. Baluns range in price depending on the job they need to do. A simple video (yellow) and audio (red & white) balun pair might cost less than £30. At the other end of the scale is the Balun pair for converting HDMI and also carrying remote control signals back down the same wire. Those might cost £200.
Video & Audio over computer network
This is a true network. The AV signals are converted in to IP packets that can travel via a router to various distinations inyour home. This sort of system would integrate with your existing (or new) computer network. One socket on the wall in a bedroom could give you your broadband connection, access to your home server, and all of the AV devices currently hooked in to the network. Ball park figures for this sort of solution range from £2000 ~ £5000+
For your cabling at home, I'd recommend you go with two decent quality satellite type aerial cables to each room, and add 3 CAT cables per network wall socket location. Put a network socket behind each TV point as well as the ones at 450mm floor socket height. Run all the cable back to the hub point under the stairs, Label Them!!!, then decide what you plan to do with them once the dust settles after the build is complete.
