They've said best efforts. That will be the contract.
So they'll fulfil based on orders received to the best of their ability. The UK ordered three months earlier and have preference, the EU have yet to even approve it. So the UK received their order quicker, when a bottleneck has occurred it's affected the EU considerably more severely.
AZ, understandably, would have used the EU plant for primarily EU production of the vaccine and the UK plant primarily for UK production, which is just common sense.
Have we seen the contract? No. But there's nothing in what AZ are saying that doesn't hold up - they aren't selling elsewhere for a bigger profit, there's no motive to downgrade the shipment otherwise. It just looks like they're fulfilling private contracts with governments, and one contract was placed earlier so was less affected, the other was placed later so is more affected.
The EU, rather than admit blame on their processes - of which there is ample evidence - is trying to claim - without evidence - that AZ are preferring one buyer over another. This isn't like making PPE, which is predictable; it's a biological product and thus is subject to very high standards, if something goes wrong with production, it goes wrong.
Also, from a purely business view, even if AZ did for some reason prioritise the UK, I couldn't blame them - why make and stockpile millions of the doses when it hasn't been approved yet, even now nearly a month after the UK? I don't think they have done that, but simultaneously as a business I don't think they automatically have to be beholden to the EU being slow.