I used a local garage for 4 or 5 MOTs with no problems (they only charge £29, probably due to local competition).
Then I took my Z4 there last July and it was failed on the O/S/F Anti-roll bar link rubber being split. They said they could fix it for £84 while I waited, so I agreed - but would they really have one of those in stock? Because I never saw a delivery from BMW, ECP or any other parts supplier while I waited!
Took my E46 there last November, and they gave it 2 advisories for rear indicator lights not being orange enough (clear light covers so it uses orange bulbs) and low engine oil level - but it has a warning light for oil level and it hadn't illuminated! Plus I had booked the test as a "while you wait" appointment and it took nearly 2 hours - I won't be going back!
As my Z4 will need a service in June I may just get my BMW indy to sort the MOT at the same time.