Stevlup
I think that’s a bit harsh on the first 3 UK companies.
What you have to remember is that this is a 2-way arrangement. From your POV, you want the work carried out. From the avionic company’s POV, they have to be willing to carry out the work for you as they want to make a profit and are probably concentrating on fitting up to date avionics into modern aircraft. They make money on supplying new equipment as well as installing it. You might ask someone to quote, they might not wish to quote as they know they won’t get the business
I can see immediately that you are looking for a cheap fit. I don’t know what you fly but you are asking someone to swap out an old redundant nav/com for a not so old superseded GPS/nav/com (which, at a guess, you will supply yourself?). For some companies, they know, without asking you for more info, that there won’t be any profit in doing that so why waste the money replying? OK, not replying is bad customer relations but maybe they really don’t care what people with small cheap aircraft think about their company. Maybe if you had a King Air it would be a different story but if you had a King Air, you would probably be installing a Garmin 1000 system.
That said, I hear good things about both the companies you have named.