Any other engine builders in the UK are not worth dealing with
Sweeping and ridiculous. There are plenty of folks out there who can build an engine just as well, if not better, than those mentioned above. I fully concede there are many who are also pretty dodgy, but to generalise like that is rather poor form.
On the matter in hand if your engine is running well why tear it down? Keep track of the compressions, oil usage figures etc. Cut open the filters whenever you change them, wash them down with some avgas and check for ferrous metal. Send off an analysis kit once a year etc.
Ours was using a fair chunk of oil and we found metal in the filter. We pulled the cylinders and a piston pin end cap had gone walkies and put a big scrape down the side of one of the barrels. Re-con cylinder on, new piston & caps, job done. Rest of the engine was in great shape so we chose to spend less than £2k to fix the issue at hand in a couple of days and she's been sweet as a nut ever since. Got 2400 hours on the lump yet it burns no oil, makes plenty of power, compressions are a good 10psi north of minima. No plans to spend money when there is no good reason to do so.