So yes, perhaps AZ are legally on solid ground, but on the other hand it's worth pointing out that EU's motives here really are not to engage in some kind of war with Britain because of Brexit and the loss of some UK tourists to the Costa del Sol, as suggested above.
The EU is now a trading rival to the UK.
At some point there was likely to be trading rivalry or disputes to arise (hence the need for there to be a mechanism within the deal for such disputes and it being a real sticking point).
We know there is a world shortage of vaccine. We know that AZ based and funded in the UK (but also a significant EU pharma company) has one of the few vaccines that is regarded as viable.
We know the EU is having major political issues over its vaccine programme.
Thats a cocktail that makes the chances of the first trade dispute between the EU and UK highly likely to be over covid vaccine. It is not any great shocker.
Who is 'in the right' is a different question, but its not unsurprising to see this situation arising. In the slightest.
If the tables were reversed, you can bet your life the UK would be trying every avenue it could to try and apply pressure for additional or faster deliveries in one way or another.
Nature of the beast. Its why the EU insisted on a single strategy to stop infighting and competition between member states (which looks likely to break out anyway). WHO warned about the possibility.