Europe has never had a meaningful regime for approving small mods like the sort of avionics upgrade referred to.
It can pay to get it all done by a company with the EASA design approval; often these have template paper packs which they just modify for the particular job and then it costs a bit less... £2000 instead of £5000
But as always the key thing is to get a written confirmation from the installer before leaving him the aircraft. Too many people have discovered something is a major mod when the plane is all in pieces. Proper project management is close to nonexistent in the GA avionics market.
The FAA has a much better system, with the IA having considerable discretion and Part 43 carries a clear enough list of Major Mods
here. Unfortunately even this stuff is widely "interpreted differently" in the UK by some firms.