If you fly under the Schipol TMA (<1500') the controllers
ask you to switch off mode-A transponders and mode-C
transponders which aren't giving an accurate altitude
read-out to them.
This applied under the London TMA might well alleviate a
lot of the problem heavy traffic has with GA under their
controlled space. That said a reasonable quid-pro-quo
for that restriction would be the provision of some dedicated
LARS for the Luton / Stansted corridor.
I'm not sure about the truth of most traffic being
transponder-less...my experience with Skywatch, an
active-TCAS system, on a Cirrus SR-22 is that it detects far
more aircraft than you would normally see while keeping a
good visual look-out. In fact it makes you realise exactly how
busy the south-east is with aviation traffic.
-- Andrew