https://www.politico.eu/article/the-key-differences-between-...
The conclusion, at least as I interpret it, is that both the UK and the EU can reasonably claim to have priority, based on their respective contracts, but that the UK did a better job than the EU in making theirs enforceable. Hence, AstraZeneca has prioritised them, while the EU has been forced to resort to political rather than legal measures to attempt redress (publishing the contract, export controls, etc.). And the UK's closer ties to the AstraZeneca vaccine project likely did mean they started work on the UK supply chain somewhat earlier than in the EU, even although the final contracts were signed essentially simultaneously (one day apart).
I honestly don't really see this as an affair with clear good guys and bad guys, but it seems like most people are determined to do so, and inevitably along the lines of their pre-existing sympathies.