LF
Elstree is a pretty good bet (its more or less my home airfield), its about a 15 min taxi ride to either Edgware or Stanmore tube, or about the same to Elstree BR which gets you into St. Pancras. Can't stress the PPR element enough, Elstree has a lot of problems with NIMBYs and ATC will deliver a sound bollocking if you don't follow the rules.
Have a good trip.
BB