Even having huge amounts of Pfizer is probably going to be problematic for the EU.
Its a question of logistics and this is where AZ wins hands down.
To illustrate the point: Italy has a limited number of vaccination sites. These are all in hospitals and this reflects storage issues. As it needs -70C it can't just be delivered anywhere. This is also one reason why UK roll out was slow at first until AZ came online. Pfizer is much more limited in where it can be administered. And when you need to vaccinate the entire population in a very short time frame thats a real issue.
Realistically AZ offers something that Pfizer can't. Even if its not quite as effective. And it'll be a reason why the EU will be extremely keen to get some even if they can scale up Pfizer production (Sanofi has now said they will produce some - why the fuck this wasn't in planning earlier is beyond me - it smacks of being slow off the mark).
I would imagine there is a certain awareness of this. Turning the tap off on Pfizer going to the UK perhaps wouldn't be the smartest move for this reason.
I think that Johnson would not back down in a vaccine trade war. Just on principle.
Also Pfizer isn't just made in the EU. I've said this before that when the Trump turned down supplies from Pfizer which then were offered to the EU which they tookup where were this doses being made? The US.
So if the UK contract is robust it may still require delivery from the US if the EU supply chain is knocked out. Given the risk of No Deal it would have been prudent for the UK to have made deals which offered a none EU supply route in emergency - my point being, if the UK was on the ball, they will have considered this.
Irony klaxon: The EU might therefore need us more than we need them.
We will see how this pans out.