There are hundreds of Chinese restaurants which serve dimsum in London - some of them are as good as dimsum in Hong Kong, others are less good but still tasty, serve food quickly and are cheap (we generally get by with the meal costing less than ten pounds per person, depending on who is there).
The restaurants are spread out through London - for sure there are many in Chinatown but also north (Baker Street), south (Elephant and Castle), east (Docklands) and west (Chiswick). It's nice to move a little around as you tend to get different people coming depending on the location. There's a base core people who tend to turn up - generally the underemployed or those working for themselves.
Organising London.pm's weekly dimsum for the last few months has been really fun: it's nice to travel around and see friends with nice food. Why don't you come to one? Check the London.pm mailing list for details - but today (Thursday 27th March 2008) we're going to the Oriental Brasserie in Chiswick.