You say you want to "purchase" an RV-10.
If you mean an already built example, then the market is almost vanishingly small. and what there is being US based.
The PFA are discouraging of imports, but if you do find one, it should be possible. You do need to jump through a few hoops though to get there, and that will only be possible after the first UK example gets approval.
Building a quick-build kit is within the capabilities of most, but will need at least 1500 man hours, much more if you start adding on the goodies.
I've found Van's to produce generally excellent kits, with good support and well thought out designs.
Join the UK yahoo rvsqn list and you should be able to make contact with some UK builders. Also the "Van's air force" web site has a dedicated RV-10 board which should be worth perusing.