Comjam, as an example, if you level at the SIDs final level on a heathrow dover departure, you will meet a gatwick dover departure at it's final level just past Detling. Similarly, SAM LL and KK's meet, as do LL CPTs and KK Kenets and so on for nearly all same direction sids from adjacent airfields.. Also, not all sids are seperated from the final approach of the airport they leave from, so although it might work in theory, the London TMA doesn't quite fit that theory.