London will only provide a radar service within controlled airspace, outside controlled airspace London Information are available for a Basic service from FL195 and below.
Not true, London will provide a radar service outside controlled airspace if workload permits AND the aircraft is FL70+. Also, to the west, ATSOCAS is available from Western Radar when they are open, and if not, sometimes London Military will provide a service.
To avoid this happening, insist you are handed over to a London Control frequency instead.
HD is correct, you can insist all you want but if you are outside CAS it is very unlikely you will get a service from London Control.
IFR may be canceled, flight plan trashed, further clearance is null and void, and you are forgotten about.
Absolute rubbish. Ask London Information (or whoever offers the service outside CAS) to request a joining clearance at the point on the flightplan where you will re-enter CAS and they will (and do every day). They are not always granted but there will be a good reason why not.
zonoma
(London Controller that provides ATSOCAS)