@CrotchBurn - not surprising. Many aeons ago, I drafted UK legislation implementing single market (and third country import) directives into UK law. This sort of non-tarrif trade barrier dressed up as something else (health legislation, "transparency" about where vaccines are destined for) is absolutely standard Commission MO, I'm afraid (and it's shit, and designed to be shit, for the third countries on the receiving end, which, post Brexit, we number among).
However, presumably this affects Pfizer exports. So in effect they're saying "the one card we have left is - give us your UK produced AZ vaccines, or we will block supply of second doses of Pfizer."
I still think turning it into a bash-Brexit-UK exercise, when it's actually a quarrel with AZ (and one where AZ has the moral high ground - they were indeed contracted on best endeavours, as far as I can see this is standard practise in the pharmaceuticals industry because of problems of scaling when you industrialize a process) which the Commission are trying to distract attention from (because they have cocked up on so many levels - not procuring enough doses, dragging their heels on approving the vaccine, all sorts of dodgy dealing over saying "we'll centralise procurement" then doing deals with the French to protect Sanofi's position, then finding Sanofi couldn't deliver the goods).
Which is not to say the UK's handling of the covid pandemic hasn't been woeful. Every big decision Johnson and his merry band of raving incompetents had to take was made two weeks late, with catastrophic results and one of the worst death rates in the world as a result. But the vaccination programme is the one thing they have got right, and the thing the EU has cocked up.
I find it interesting to read the US press for the same reason you read the Swiss media - and even the Democrat leaning, pro EU outlets seem to be pretty much "the CEO of AZ is telling it straight, the Commission is trying to use it as a distraction from a cock-up of their own making, and however much of a disaster one thinks Brexit was, this really wasn't the fault of Brexit."