I get asked this kind of question by a lot of pilots in the UK, who have been around the usual shops and either got very poor service or the shop could not fix it.
Very few avionics people in the UK are electronics engineers.
Far better to go to America.
If you email me, I can send you some PDFs from U.S. pilot forums where various recommendations are found, and I know people who have found this very helpful.
However if you need work done in situ, then you have to get somebody in the UK.