You ask for a UK testing company. If it is for ancestry purposes, and you are looking to test autosomal DNA, I wonder if there is one with a large enough data base. Here's my own experience with a US company and my thoughts about what I'd do today.
My father, myself and my husband were tested by 23 and me some years ago. Initially it was for health related DNA but of course using the result for ancestry is part of the package. Testing and getting results went smoothly. It all worked well for a year or so but then there was a hiatus whem they migrated to a new platform. They moved test kits in batches.Those from the UK were left a bit in limbo as they were amongst the last to be migrated. It was hopefully a one off and its fine now.
However, I wouldn't use them again. We have lots of matches but very few of any size because relatively few testers are from the UK. Most of the matches are from the descendants of aunts and uncles who probably emigrated in the 19thC. Very few of the people that match have developed trees so establishing a connection is almost impossible. Having said that, I have found one distant cousin whose ancestor went to the US in the early 19th century. I've managed to establish links with a couple of maternal 2nd cousins whose family remained in the UK. I've not established a single connection from my father's side. Husband has found a couple of 2nd cousins, one in Australia, one in Argentina.
If testing again, I would probably choose Ancestry.This is also an American company but it has in recent years advertised widely in the UK and I think (but don't know for certain) that it has a larger pool of test takers from the UK. It's focus is of course ancestry.
If you feel you definitely want a UK company an option would be Find my Past which is UK based. Unfortunately, its only been DNA testing in the last year or so. Most of its testers are likely to be English but I don't think it has had long enough to build a large data base . This may well change so it could be an investment for the future.
