UK Dim sum
In my opinion, the best in London for Dim Sum are now:
Phoenix Palace on Glentworth Street near Baker Street
Royal China Putney, SW15
Plum Valley, Gerrard Street Chinatown.
In particular, the 1st two have few tourists and instead are mostly frequented by local asian families.
However, none of these restaurants operate a trolley service. The two in Chinatown London that do (New World and the other one on Gerrard Street) are of poor quality, in particular New World's thick Cheung Fun.
The only Dim Sum restaurant in the UK with a trolley service that I would recommend is Thai Woo in Manchester - a terrific place that is not only huge but very busy with the local Chinese community.