With apologies for posting already on another thread...
It's all very interesting, but obviously no one has seen both contracts other than AZ, so really who knows. I would be VERY surprised if the CEO of AZ didn't have a strong basis for this - he will have had lawyers crawling all over the agreements weeks ago before breaking the bad news to the EU, so the relevant clauses should have been very well studied.
Clearly, "best endeavours" could mean lots of things in this context. In English law, it doesn't require you to break a binding legal commitment with someone else to deliver something where you are legally committed to that third party to do so (ie not best endeavours), so if that is what the U.K. has, it does get priority over "best endeavours" to someone else. Whether, if this is the set up, the EU understood this at the time (although clearly Soriot suggests that it was understood that the game plan was that supplies would be made from a continental factory at least initially) is a clearly a live issue.
The export of vaccines to the U.K. from the EU factory is an interesting one - I have no idea whether AZ decided they needed to go to the U.K. because the EU hadn't approved the vaccine yet and there might be storage/distribution issues, because the U.K. has absolute priority in its contract or whether this looks more like a breach in retrospect because they couldn't deliver to the EU - 4m doses was quite significant to the U.K. obviously, but not so much to the EU really, so it isn't that relevant to the main issue at hand in terms of the delays on the wider contract for far far more doses than this 4m. Bit of a red herring in the grand scheme of things these initial doses.
I am a remainer and have no real beef with the EU (from my dealings with them as a lawyer, it is pretty bureaucratic and needs reform, but I thought it was far better to stay in and try to advocate from the inside) but I do have limited sympathy for a party who really took their time and invested less in order not to take the risk of backing a loser (but backed the french vaccine anyway for political reasons) and then expects to be able to obtain the same level of priority. Risk and reward are usually linked. But sympathy is not really relevant - it is the terms of the contract that really count. We just don't know!