I recall in the early days of ops into City from the west country that London Control wouldn't allow them to descend west of Heathrow; they had to go via OCK and then descend to the east of BIG; this happened with outbounds to the west too.
It was proposed therefore that Farnborough would work the traffic when it was approaching their area and descend it below CAS to transit to City at 2,400ft with a handover to Thames Radar.
Thereby began another problem.Thames Radar for London City traffic was operated by controllers who were initially seconded to them from other units because Heathrow couldn't/wouldn't handle the extra workload of vectoring into the City CTR.
The Thames Radar controllers weren't allowed to wait in the Heathrow part of the ops room and had to wait in the rest room while there were no City arrivals or departures until an aircraft was due then they had to go to their workstation and open up the operation, shutting down as soon as the aircraft was either remaining on the ground or it had departed. Of course this situation was eventiually solved when City traffic got busier and Thames Radar became more or less permanent
Nowadays of course, City Tower and and Thames Radar are both operated from Swanwick; there are no controllers at London City any more.