I think the main point of that particular argument is that folks in the UK claimed that "they came first" while EU reps said "doesn't matter who came first, what matters is what the contracts say". (I tend to agree with the latter)

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.