Paul Rice
As you pointed out, a state does not need to be an EU member state to be full member state of EASA, which all of the mentioned states are. And of course there are many other levels of agreements possible, yet, the UK decided to have no agreement whatsoever. No bilateral agreement, no technical cooperation, no memorandum of understanding and no working arrangement. And least of all, there is no EASA representative office in the UK, and no UK management board observer at EASA. Which means the UK has a more distant relationship with EASA than countries like Singapore, India, Thailand and Kazakhstan.
The idea of a professional licensing institute sounds interesting, but of course that would require the UK government to relinquish regulatory control over a professional sector, and the EU to accept it. For most professional qualifications to be accepted from the UK into the EU, it has to be done in each single EU member state, with the exception of certain professions that are EU regulated like pilots (sic), ATCOs, lawyers, seafarers and others. In the end an approach like that could de-politicize the whole issue and make it easier for both sides.