I've just been referred here from your other post. I'll do my best to answer your questions. I'll say again, I hope you have good time in the UK, British people are a bit funny about Americans (jealous, I suppose), but there is a great friendship between our countries. Oxfordshire has some beautiful country and great pubs, though I don't suppose you'll take to the beer!
1. The A-Class has been a real success for MB (in terms of sales and profits) despite a rather difficult start (the first models would roll in extreme conditions). They are not well-regarded by the motoring press, but the sales figures speak for themselves. I have driven a couple of A140s and they are a very good car to drive, albeit they do not reward being pushed hard. They are very spacious internally, supposedly as big as an E-class. this is because the engine and stuff is under the floor. There is also the Smart car, which you may have seen knocking about, this is even smaller than the A-class and has a MB 600cc three cylinder engine.
2. The UK (and Europe) has always had smaller engined cars than the US. I don't know why. You'll have seen how horrifically expensive petrol is. When I was a boy (about 15 years ago), a 1.6litre was regarded as a big engine. I remember we were all thrilled when my dad got a 1.8! Engines are getting bigger these days but as you've noticed, there are some models here that you don't get in the US. In the W201 and W124 series, for instance, these engines were available in the UK:
In Europe they even had a 200D, which must have moved at a glacial pace.
3. As for any good shops, you're a long way out of my area (in UK terms). You should try to pick up a copy of Mercedes Enthusiast and have a look at the small adds. There is a chap in Birmingham called Andy Gayle who specialises in MB cars, he was the subject of high praise in a recent feature. His number is 0121 558 4848 and his website is www.mercedesservicing.com
. Birmingham isn't too far from where you are based, about 1-2hours depending on traffic, but there isn't much about the city to recommend a visit, it's a bit of a dump!
4. Birds is the only tuner I can think of off the top of my head. Barbus seems to have an office in London. Tuning isn't the big thing here that it is in the US or Germany.
5. You should find a breaker's yard specialising in MB or German marques in every town. Buy a copy of Autotrader and look in the back, where there is a section on breaker's yards. Also, there are several mail order outfits advertised in the back of Mercedes Enthusiast.
6. Finally, look out for speed cameras! Good luck.