After figuring the above out another wakeup call for me was being treated like persona non-grata by Solent radar (no surprise there) at night in IMC having filed an IFR flightplan from London back to Plymouth. I had no idea that they were not aware of my flightplan.
Yeah; you discovered another UK airspace secret
IFR flight plans filed below the magical "enroute airspace" go basically nowhere enroute, and no IFR clearance is maintained. A lot of people have been caught by that. Solent is just a bit of Class D around an airport or two. To get a continuous IFR clearance in the UK one needs to file for something like FL120 or higher, to be sure it will "stick".