Also, the US signed a deal with AZ in May of 2020, which would seem to put them ahead of the UK and EU? https://www.fiercepharma.com/pharma/astrazeneca-scores-1b-fr...
There's some minor amount of irony if the UK contract really was a day later than the EU's (in a 3 year old screaming "gotcha" kind of way), but it still shouldn't change a thing about who gets how many vaccines at which point in time.
In the end it seems to boil down to AZ not meeting the numbers they promised and having to cut corners _somewhere_, and the EU doesn't want to be the one bearing the load - at least not alone: other vaccine suppliers cut down deliveries due to production issues as well (e.g. biontech/pfizer, also due to retooling a Belgian factory), but they split it across the board and apparently handled that transparently with all customers. What goes around comes around, and so that was a pretty painless experience for all involved.
With AZ there was the additional complication that the company is part British (and it's generally presented as a British company only) and those contractual disagreements happened in parallel to those other contractual disagreements between the EU and Brits about shipping goods, so there was already a higher base temperature in the public discussion of the AZ delivery schedule.
Later, UK and US agreed to buy AZ vaccine and are paying 3$ and 4$.
https://www.bmj.com/content/372/bmj.n281
Now all of the sudden agreements with UK and US are fulfilled and with EU are not.
Countries are currently outbidding each other and stealing the supplies. It was naive of EU bureaucracy to think that the cheapest price is the priority.