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
OR
Contact a London Amiga User Group...
http://web.ukonline.co.uk/ant.london/ Email address....
ant.london@ukonline.co.uk
OR
South Essex Amiga Link
http://www.seal-amiga.co.uk/meetings.html
They are in Basildon.... almost London and seem pretty active still.