The issue as i understand it is this mate.
Both the Uk and EU have contracts with AZ. The contracts are similar, both based on best efforts and the EU one was actually signed the day before the UK one. The contracts mention the supply chain for both countries as plants in Belgium, Netherlands and two in the UK. Both the UK and the EU paid for the development & manufacturing etc of all four plants.
My understanding of the issue is the EU's order has been left short by 70% by AZ, who arent exporting from the UK plants - the inference is an export ban by the UK government. The UK's order has been fulfilled 100%.
10 mill doses of vaccines have left EU plants for the UK. 0 vaccines have been exported by the UK.
Essentially what the EU is inferring is that the UK has a defacto ban on vaccine exports, keeping what is produced in the UK to ensure their order is met. What the EU has inferred today is if the UK continue to do this, then they will respond in kind and stop all vaccine exports until their orders are met.
The same thing but more overt is happening with the USA, they have no problem acknowledging it though, but they are less reliant on EU plants.
To me and the bit ive done looking into this, the fault could be AZ's, seems they entered a duplicitous contract arrangement with the EU and UK, the bind is AZ and UK have been happy to collude on exports, each hiding behind the other unofficially.
As for the surplus, i dont realistically think there is one, Pete posted figures on it yesterday and i showed him that the over hang of unused vaccines matched almost equally the number who received a first dose, defacto they are stored second doses - most EU nations wont give a first dose unless they have the second dose.