There was no production capacity 12 months ago. EU suppliers failed to upgrade and improve yields sufficiently in time. The UK/US started expanding right at the start of the pandemic, so effectively had a 6-8 month head start.
Easy to say but not necessarily true. Let's look at the Halix factory, which is the main point of contention in the EU/UK dispute. The Dutch government (thus, EU) already offered them a major contract in March/April '20, for precisely this purpose. They already had started expanding in Feb '20 as part of their production for the AZ trial, so they declined.