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.
10540
The problem lowish level is the London area and all the approaches and departures from Heathrow etc.
Chances are you will be vectored all over the place and dropped down so your IFR planned route goes south back to brussels anyway
I had the same problem with London! I ended up being vectored on a tour of southern England, passed from one controller to another till having gone towards Birmingham I ended up being vectored back south west to CPT when I was supposed to be going North west.
The controller hadnt a clue where I was going and then dumped dumped me to London Info at FL100 on top of an overcast
So not that unusual! We are unwanted but tolerated small fry in that area.
Pace