Your article from may doesn't say anything about signing contracts, only that they are ready to go.
I've seen the articles mention may, june and august as signing dates of the contract, so clearly there is some 'miscommunication'. And both the may and june date originates from AZ, while the actual date turns out to be august.
There are plenty of articles from May 2020 that all discuss in various words that a deal was struck. If you believe that AZ didn't sign a contract with the UK before Aug 28 then the burden is on you to prove that.
No, there are plenty of articles from which you deduced there was a contract signed by the UK before august.
It's impossible to prove there wasn't a contract signed, it is however possible to prove a contract was signed. Your articles never provided evidence for the date the contract was signed.
I've seen the articles mention may, june and august as signing dates of the contract, so clearly there is some 'miscommunication'. And both the may and june date originates from AZ, while the actual date turns out to be august.