In London:
Folk in the Cellar
Folk on Monday
Morris Folk Club
Sharps
The Harrison
Back to the Folklaws
Chalk Farm Folk
Duke of Wellington, Dalston
Princess of Wales, Primrose Hill
For concert goers also:
Kings Place, St Pancras Old Church, Cecil Sharp House and Green Note

Cellar Upstairs was mentioned in the older part of the thread but has since moved (twice).