This "getting dumped out of CAS" has been a hot topic for a lot longer than I've had the IR, so I can't offer a historical perspective, but I get the feeling that things have improved in recent years.
Apart from the Paris Control --- London Control business, which seems to be very current. You won't get a handover from Lille etc straight to London Control, to continue with the IFR clearance, even if the flight is 100% in Class A in UK airspace.
I don't do much airways IFR in the UK but my experience of it has been reasonable. Lots of vectors, sure. But at FL100-120, OK.
However I must say that I usually file FL140/150, simply because the routings are often so much better, my oxygen comes cheap, and there is no possibility of anybody thinking a FL140 flight plan is from somebody with an IMC Rating, which is a possible interpretation of filing at say FL070.
But getting dropped anywhere near CPT at FL100 is astonishing incompetence and I would simply refuse because there is no way to get down from there without conflict with LHR/LGW traffic.
OTOH getting dropped @ FL100 OCAS, which is possible slightly SW or NW of CPT (Class A base e.g. FL105) would be in line with London Control's working practices, but they should not have sent you OCAS in the first place. I wonder when this happened, and was it MORd?