Not sure I agree.....The airspace was not devolved nor has it been closed. Aircraft fly over countries without permission to land all the time. If I fly London Perth I fly over many countries with many lockdown rules I am ignorant of but their airspace isnt closed.
So the Welsh authorities have no jurisdiction over my flight.
If I have an emergency and have to land, it was not intentional and Force Majeure. You cant be prosecuted for it in any case, but even under the Coronavirus Regulations there is no rule that is broken as you 'left your home' in England which is legal. The Welsh police are stopping people not for driving along the road but for travelling to somewhere in Wales for something that isnt lawful ie the purpose of the journey in Wales. I wonder what would happen if you told Officer Plod you were just going for a 'run' and not stopping in Wales....