I gather from comment on the news that the AZ contract with the EU mentions four production sites, two of which are in the UK.
It's all just speculation unless these people have seen the contract and even if true, it doesn't necessarily mean they have to use the UK sites. The contract might say (as an example, obviously, I haven't seen it either) "the contract will be fulfilled from site e and u and AstraZenica
may use sites g and b to make up any shortfall". In that case, AstraZenica are not obligated to fulfil EU orders from Great Britain. Change "may" to "must" and it's a different story.
I doubt that the AZ contract with the UK would be substantially different to that with the EU in terms of 'best efforts'.
That's just speculation too. There's only one group of people that have full knowledge of both contracts and they seem to be of the opinion that AstraZenica's best interests are served by throttling EU supplies but not UK supplies.
As an aside, the contracts manager of the first company I worked in said "never sign a contract that commits you to 'best efforts'". He would always insist on it being replaced with "reasonable efforts" because the former commits you to fixing everything even at the cost of bankrupting the company.