It all depends on what is in the contract, the UK bioreactor is producing what is required - whether by luck or good management by the technicians, whereas the European division is not yielding the same quantity it is capable of, and thats what is causing the issue, if the contract states its for the European division to deliver a set amount then they can only get what its producing, if its based on a company wide system or 'the AZ' company standard contract, then they could claim its a breach of contract and the standard conditions come into effect - paying for the product does not mean you are entitled to demand the product, as the product belongs to the company until it delivers it, only the company can divert the product to another customer.
They could take a hit and call their bluff by repaying the EU money with interest and say that as they have not delivered, and that the contract is now invalidated and therefore the company will not be supplying the product, that would leave the EU in very sticky position relying on Biontech and Maderna alone, and therefore inoculation rates would fall further, one of Germanies advisory bodies stating its not found to be effective with over 65's is as good as any reason to cancel the contract on it not being suitable for purpose.
AZ India's produced version could be shipped to the EU from AZ, but the countries that are buying that divisions product might complain as well.
The EU's interference to the purchase system is what has caused the problem (the individual countries were going to buy from the companies but the EU said no - we will order it all as one purchaser - with two months rolling by as a consequence), thats why i voted for leave in brexit, an un-elected french designed system that stamps on the sovereignty of all its members under its guiding principles.