I'm in the same boat at the minute my rear calipers look horrific
From research https://www.bcs-automotive.co.uk seem to be very highly thought of. I have a quote from them for £330 for all four.
the issue for me is finding a local garage willing to take my car in take the old calipers off, ship them, keep my car and then fit them again when they are returned as they are a mail order service only, albeit they say 24h turnaround.
For replacements https://apecautomotive.co.uk/ who seem to be highly recommended as replacement pads now have their black series for our car which are replacement calipers in err black! I don't know anybody that has the calipers though.
For ultimate replacements https://www.pbbrakes.com/ which a quite a few owners have gone down the route of, they look gorgeous, however aren't cheap, you won't get much change from 4k for a decent setup.
Lastly there's always a rattle can from Halfords i guess if your happy with that approach