Lucky sod!! How did you find land in the UK?
Anyhow, first suggestion: lots of plastic ducting.
Second suggestion : build a bungalow (or have suspended floors with a 3' crawl gap - all your problems go away and you can run services and all kinds under there.
Third suggestion: I suggest designing it
Seriously.
And that means essentially knowing what you're getting and where it's going. The ducting helps with "oh sh!t, I needed a DVI->HDMI cable going from A to B" but you really need to get most of it right at once (unless you like your lounge looking like a patch cabinet!)
You'll want a place to keep servers - usually called a "node zero" by HA types.
Then think about:
* where are the speakers going? In ceiling? On walls? On the floor?
* where are the amps going? You don't really want speaker wire running around the house - bad for the signal. And going from the node0-(cat5)->room-(line level)->node0-(speaker level)->room is horrible.
* where are the screens going? Walls, under the kitchen units?
* how do you get media from servers to device to amp/display?
If you have a cinema room are you projecting or plasma'ing? Projectors can be built into vented cabs. Screens can be built into the ceilings *if* your joists go the right way (mine don't but coving is great!) Plasmas need strong walls.
Are you going client/server?
Also, think very hard about room layout - a huge problem is having windows and doors on every wall.
Ideally you want a room that has blank end walls, the door in the middle of one long wall and the windows in the other long wall. Then you sit backing the short wall and project onto or hang the plasma on the other short wall.
Don't neglect other wiring: especially security systems. Wireless is great in a listed building but having the batteries go flat in my security system sounds a bit dodgy
You may have guessed I run a server room (study) with MythTV on diskless PCs in the bedroom and cinema room and
squeezeboxes in the
bathroom and other lounge. I have cat5 to the kitchen just in case...
I've done lots of remodelling so re-running the cable wasn't too bad.
I've done the alarm but still need to wire up security cameras (just IP webcams).
Have you seen the UK selfBuild group?
FAQ here:
http://www.sbfaq.borpin.co.uk/dokuwiki/doku.php?id=startSeriously essential info
There is also a UKHA group - which is fine it you like spending money on X10 and bow down to the church of Redmond
See the community links from:
http://www.automatedhome.co.uk/Personally I'd go MisterHouse - but then that's because Larry invented Perl.