But to deny that such a contract exists, or that that contract could affect the supply to another customer is deceitful, and would entail a breach of contract.
If that other customer is another government, and denies that such a contract clause exists, what does that suggest? That one or both are not being honest.
The supply to UK from EU plants occurred prior to AZ seeking approval from EU (EMA) so your question is invalid.
The EMA approved the use of AZ vaccine two weeks after approval was sought by AZ.
We don't know how long UK took, but it looks like it could have been several months. (UK started cooperating with AZ in September and approved the vaccine on 30 December.)
AZ have not yet sought approval from US (FDA), so any argument about shortfall in US, or from US is invalid.
AZ are not applying for approval from US until Spring at the earliest. And the US FDA process is different, in that they conduct their own tests, unconnected with the suppliers data.