It much depends on what you want to do.
Few if any UK avionics shops actually understand the equipment beyond following wiring diagrams in the installation manuals.
Anybody can wire up a spare circuit breaker leading to a connector for a yoke mounted GPS.
But if you want some major work done, there are very few firms you can go to who will do a decent job, and you need to go to one that has installed the exact equipment before.
I've had several jobs done, all by "highly recommended" firms, which were not fully functional and I had a choice of keeping it, or constantly flying back to certain airports so they can "have another look" at it.
An example of what happens is a radio altimeter installation which is supposed to connect to the landing gear and warn if the gear is still up when you are below say 200ft or whatever. They get the radalt working, but give up on the warning bit... you still get billed the original price because they offer to "have another look" if you come back
This is why for every person who will recommend a particular firm you will find another who will slag it off.
Sorry to be negative but I've had rather a lot of hassle on this front.
£1400 for a GPS wiring/antenna is ridiculous but might not be if there is a lot of work in the antenna mounting. Under FAA rules (N-reg) you could easily spend £1000 just on the DER drawing approvals for drilling the roof. On G-reg people tend to just drill the holes...