Originally Posted by
custardpsc
Sure you can. It isn't always easy because traffic issues around, for example, Heathrow may cause London to have to descend you out of controlled airspace, you may end up in IMC in class G without any radar service
Or they may transfer you to one of the 3 radar frequencies operated by Farnborough for low level IFR and VFR traffic under the LTMA.
Back in 1969, I remember a guy in a Cessna 150 filing IFR from either Luton or Stansted to Brussels requesting FL90. As it was for a departure at about 0200 UTC there wasn't too much other traffic about so it should have been possible, but the controller (JR) who issued procedural clearances obviously didn't agree with this and issued a clearance at FL90, but to maintain not less than 150 kts, clearly impossible in a C150.