I don't know exactly which clubs are at each airfield, but you can def do better (as in closer) than Southend. White Waltham has a huge club and Denham has one so you could take the Heathrow Express to Heathrow from Paddington and then a taxi from there to either of those. Or go by tube to Elstree (not as quick). There's Stapleford as well, but I don't really know how to get there. Damyns Hall is on the overland or tube at Upminster, but they only have lighter aircraft.