What about searching for established Greater London computer fairs and using the same venue/hooking up with them?
I found one as an example... http://www.wizardfairs.com/index02.html
Contact a London Amiga User Group...
Email address.... firstname.lastname@example.org
South Essex Amiga Link http://www.seal-amiga.co.uk/meetings.html
They are in Basildon.... almost London and seem pretty active still.