I think that Stick gives a rather optimistic view where importation is concerned.
If you bring an N-registered aeroplane in, that holds a full CofA it should be relatively straightforward to move it onto the UK register. Even easier however is to get an FAA licence and fly it on the N-register here, something that is common and perfectly legal (although I'd have a word with somebody who has done it before, there are one or two pitfalls).
If it is operated in the US however as an "Experimental", which means it's homebuilt (legally, if not in reality) then there are all sorts of hurdles to bringing it in. It is possible - usually! - but frankly the time and cost of that will negate any advantages that you started with. I'd not recommend even attempting any import route with an "experimental" unless there was absolutely no other way to get an example that type (or something similar) into the UK.
G