The Rathbone Place branch of Sweatshop does the follow runs each week
Monday - 5k (approx) at any pace, average 24-30 minutes.
Wednesday - Group interval training for all running levels. This session is joint with the local fitness first branch.
Thursday - 5k at any pace and 7k at about 40 minute pace.
The runs generally head through the side streets just North of Oxford Street and then into Regents Park for the bulk of the session. In my opinion Regents Park really is one of the nicest places to run in London.
So far I have only run with the Monday group, which had about ten runners.
(I'll update this post once I have run a few more times)