".....there is by far insufficient radar coverage to do the monitoring."
At present that may well be the case in many areas.
But were Mode S to become mandatory, particularly when linked to GPS and downlinking via ADS-B, it wouldn't take rocket science to come up with remote ADS-B reading heads to track and charge you.....
The FRUIT and garble which the CAA claim would cause RF congestion is
NOT a significant problem at lower levels - particularly at 2-3000 ft in the open FIRs.