How long is a piece of string....
Although the ICAO CofA standard in any country is satisfactory for overflight, for re-registration most countries (the UK included) have conducted their own approval of the type, and will have their own build standard.
You've not stated what type you're looking at, and perhaps you don't know yet, but in any country the build standard is defined in a document called the "Type Certificate Data Sheet" or TCDS. I'd recommend getting hold of both the UK and foreign TCDS for the type you're considering and looking at the differences.
What will then be needed to satisfy UK CAA will be:-
- Confirmation from a competent person or organisation (LAME, JAR-145 or JAR-21-JA company usually) that the aircraft has been modified to the UK approved standard.
- Confirmation from a competent person or organisation (LAME or JAR-145 company) that the aircraft is now fully up to the maintenance and documentation standards of a UK public transport CofA
- De-register in the current country of registration, re-register in the new one.
This is all off the top of my head, and I don't work for CAA - so I would confirm this with the local CAA regional office. Is it cheaper than buying in the UK? - no idea, only you can work that out when you've confirmed the size of task for the individual aeroplane.
G