Your build seems quite smart.
- Campagnolo parts are very cheap in the UK versus the USA. Buy from Ribble or Merlin etc.
- Carbon seat posts for cross are a bad idea due to mounting/remounting. I'd get a light aluminium one instead.
- Zepnat and Paul Milnes Cycles are UK based cross specialists. They both sell their own framesets, as well as Alan. Sonic Cycles sells Intec/Fort frames.
- If you're serious about saving weight, ditch the Nutrak tubes for Michelins or similar. Nutrak tubes tend to be pretty thick in my experience.
- I'd recommend going with the Campagnolo chainset rather than an FSA one. If you buy a groupset (this is usually cheaper than buying individual components) you'll get on with the group anyway. You can use a suitable TA or Stronglight ring to get you a nice set up with the lower gear you want (eg 34-44 or 36-46).
- I don't think you would be able to build it with Rival within your allotted budget, but I can't think of any bargain basement sources in the UK. You might be able to do Veloce, 105 or Centaur if you shop well.