I can remember working a couple of these flights in the early 90s. From memory Concorde departed EGLL then BPK - CLN - GABAD ( think that was the name but these have all changed over the years ). It then turned over the north sea on UA37 towards DANDI and once steady on that route accelerated supersonic. I cannot recall what the flight planned level was I am afraid.
It did not really cause any issues in the ATC sense as once going supersonic it climbed well above the normal levels on the north sea routes. Please excuse the reporting point names as they are probably historical now and old charts would need to be consulted to see the exact route mentioned.
Hope this is of some help
