Let us examine what we know:
The UK signed the contract earlier than EU, way before the clinical trials were conclusive. A big risk, but their gamble paid off. They're perfectly within their rights to do so. Morally and politically it's been a big boost for them. It has been their only right move so far.
The UK set in motion an ambition and a process to use the vaccines as soon as they became available and as fast as possible. Again, a bit of a gamble, but it's paid off handsomely in moral and political prestige. So another big boost to the prestige of UK government. Their second right move so far, and possibly recovered the poor rating of the government over the handling of the pandemic.
The EU signed their contract later when the clinical trials were well under way, so less of a gamble. Their contract was for massively more vaccines and value than the UK contract. Less risky and still well in time, and the delivery schedule could not really benefit an earlier signing.
The EU (EMA) approved the use of the vaccines rather later, but again, there was no rush due to the contracted delivery of the vaccines.
When the vaccine delivery were due, AZ announced a problem with the delivery schedule, eventually blaming the contract with UK for the restriction.
EU kicked up a right stink over it, using such public pressure as pressing for the publication of the contract to reinforce its argument. AZ resisted that publication. EU threatened legal action, even threatened public political action, and invoked a procedure to produce transparency over the export of vaccines. The contract was published, albeit in redacted form.
UK denied such restrictions in their contract with AZ.
UK government became magnanimous and started publicly offering to assist other countries with the supply of vaccines.
AZ agreed to supply more vaccines to EU.
Situation resolved.
End of factual sequence of events.
There is a rather egregious explanation of those events.
UK did indeed put pressure on AZ (political, moral or financial pressure) to divert the vaccines due to EU to UK. The UK, of course denied this.
The EU threats were of such magnitude that the UK could not maintain the pressure on AZ. AZ relented, and agreed to supply more vaccines to EU.
Now in this explanation and scenario, who caused the problem?
the EU are still short 40 million doses so its not situation resolved,