I would suggest a Superior engine kit that can be assemble in the UK. Then you would have an engine that some of the problems have been ironed out and you would know how well it has been put together. I seem to remember an article in either the Pilot or Flyer from a few years ago, I think a RV builder did this with guy in Wokingham, Berks putting it together.
All the problems I list have happened over two engines some several times.
The last was oil control rings worn on 3 and stuck on 4 with high oil consumption.