Well if you look at the key paragraph i highlighted in the times article and then read this twitter thread:
twitter.com/barristershorse/status/1353905785651273728?s=21
It gives a slightly different version of what might be going on.
I don't know whether the above twitter thread is accurate, but if it is, it - like the Times front page article - suggest the EU screws up on the contract wording in failing to insist on the right thing: guaranteed delivery.
The optics of the uk happily rolling out whilst the EU doesn't is a domestic political disaster for all concerned. There is a lot of incentive to throw weight around as a result.
Why the wording for AZ contract might be like that is another matter. Did the UK government insist on it when funding the vaccine? Possibly. Indeed probably. It would explain why the UK didn't want to join any vaccine supply sharing scheme.
I suspect what is 'fair' and what is 'legal' will be hotly debated.
If that thread is right, it makes it look like the EU have screwed up massively on contracts.
However this might well backfire on the uk in practice. Especially if AZ isnt as good as hoped and the EU block exports of other vaccines (which they are extremely likely to in the event of a shortage). Then the UK government has a case for legal cases over breached contract depending on how they are worded (Pfizer also has manufacturing capacity in the US which may still come into play here).
If the EU pushes back hard on the UK on this one, that might push us harder towards the US... (that's not necessarily a good thing for the EU or UK).
The more i see and read on this the messier it seems to get.