Midland VW are pretty good. I have used them to do some work on my cars a few times over the years and had them map a Passat a few years ago with good results. They are a Revo dealer. http://www.midlandvw.co.uk/
TRP Performance in Walsall Wood are an APR dealer. Had a custom exhaust done on my TT at TRP and spoke to the guy who does the mapping and he's on the ball. They have a decent 4wd dyno there as well. https://trp-performance.com/