I think a simple review of where the schools in this area do their training could make a major contribution to safety in the corridor.
Look at it this way. Almost all traffic in the North London Corridor is either flying East/West or is going up/down the Lea Valley. At present, the only other traffic is going to be students, doing stalls, 60deg turns, PFLs, climbing and descending... all in the UK's busiest bit of sky and with a low ceiling.
Such a move would also cost nothing as there are open areas a few miles to the east and west and could be implemented this weekend.
Fitting transponders will achieve nothing if there is not also the budget and resources to provide a good RIS in the area. And there isn't.