The expectation that one ought to get premium service for a premium car is not unreasonable.Sorry to hear about your experience. Two points to note...:
1. 'MB dealers' aren't one entity... they are different garages belonging to various chains (including MB UK). There are good dealers and there are bad dealers... I had poor service at one London dealership, so took my business to MB Brooklands, and after 3 services there I can't fault them (as it happens, both dealers are owned by MB UK).
2. There are good independent specialists, and there are bad ones... like with almost everything else in life. Incidentally, I (and many others) had very good service from forum-sponsors Wayne Gate in Harrow and PCS in Horndean.
But I do understand the frustration - there's an expectation that any MB dealer will provide exceptional service... sadly it isn't so.
Sadly it is not universally delivered , although it seems to be more of a UK thing , going by my very limited experience of visiting dealers in Germany , combined with reports from others , it does appear that dealerships in Germany do deliver better service .
Maybe the German mindset of doing everything by the numbers ...