Popped to T3 early just in case there were any issues.
Agent wanted to see my PCR test and vaccination proof, and boarding pass for LHR-SOF issued. No problem
Still not sure if you require a PCR to transit Bulgaria, but I would rather get the test than be refused boarding as an agent interprets the rules one way or another. Works for my onward US flight as well.
Verifly does say if you're transiting, all you need is to be fully verified.
Added: busy flight. At least they boarded children and assistance first, then just group 1! First time I've seen anything other than groups 1-3 at LHR
Got to semi-snowy SOF on time at gate 7. Decanted straight into the general areas.
Walked round to the podiums at the entrance to a holding area, scanned my boarding pass and the scanner went red. Agent dabbled a bit on the computerand it rescanned green.
Lots of Group 1 here. All very eager to board.
Arrived at LHR 30 mins early and no ground crews aware of us arriving. Might be slightly blocking a Virgin A350-1000 at present.