Ah aeropolitics. My favourite subject.
If they use an EU (say Maltese or Portuguese) AOC and registry, their chances of doing UK to the US (and indeed a whole load of other third countries) goes the way of the woozlum bird.
Let’s assume Malta. They can happily fly from anywhere in the EU to anywhere in the US (there’s an open skies agreement). They’ll need to get a foreign ops permit from the US but these are rarely unreasonably withheld.
What they can’t do - since Brexit - is to treat the UK as a stop or a base. If they use it as a stop - say MLA-LGW-EWR - that’s a fifth freedom route. And they are a no no with the UK and the EU since Brexit.
Also, if they based a Maltese registered aircraft in LGW and flew that to EWR (and going back to MLA now and again to blow the tyres up) then that’s a seventh freedom route. If you think they’ll have no chance of a fifth freedom, the chances of a seventh freedom post Brexit are about one billionth of that.
Does no one in these crazy projects ever do any research?