The London Steampunk & Dieselpunk Club (L.S.D. for short, yes I know, I thought that as well) meet regularly at The Salisbury, 90 St Martins Lane, London, WC2N 4AP (that's next to Covent Garden).
We have our own area (or 'snug') in the pub.
Meetings are roughly every 3rd Thursday.
Attendance varies from 5 or 6 people to 20 or 25 people.
They have a Facebook page.
https://www.facebook.com/groups/LondonSteampunkDieselpunkClub/