Dunno where you are in the UK, but even from France I always send my difficult stuff to these people, in Beds:
https://vintage-engine.net/They've resleeved cylinders for me (AMC twin engines), done standard rebores, loads of grinds, head work (AMC, Norton and Beesa), alloy welding repairs on 'thrown-rod' crankcases, etc etc. Also dynamic crank balancing. Can source pistons etc or will bore to suit tolerances of customer-supplied.
Superb family-run full-service engine reconditioning business, in my opinion.
As to pistons, I've just fitted IMDs in my A10 - see recent posts on here for a few comments. Not sure whether they have pistons and/or liners for A7s though, as not mentioned on page where A10 bits are listed.