CNN reports the UK's contract with Astrazeneca was actually also signed in August - like the EU's just one day prior - and uses the same 'best efforts' language.
It goes against the reasoning for prioritising UK deliveries given by its CEO in an interview https://amp.cnn.com/cnn/2021/02/17/europe/uk-astrazeneca-vaccine-contract-details-intl/index.html?__twitter_impression=true

It helps us understand why the EU reacted so dramatically to that interview by Pascal Soriot. The contract also said EU factories would supply the UK, contrary to the claim of separate supply chains he put forth. But we still don't understand why Astrazeneca took this approach.

The EU is now working on new contracts for booster vaccines tweaked for Covid variants.
Pfizer and Moderna clearly first in line - mRNA can be updated fast, and von der Leyen said yesterday it makes sense to work with existing contractors and experience with "most" had been good.