Sounds like an ATC c0ckup to me. Possibly, something went wrong with the flight plan. Let's say Calais gave you the wrong squawk; London Control would have never seen your plane on their system and after a while (30 mins?) would have binned the flight plan.
There are, I gather, other circumstances which would cause LC to bin a flight plan, but I don't have any details.
At FL120+ I have never had a problem myself.