The UK was aware of geopolitical issues and how that might impact on the UK's vaccine supply.
They had protections written into the contract to ensure that it wouldn't be damaged by those.
One of those turns out to be the slow ordering of the EU, which means that global production of vaccine was slower. Meaning vaccine shortages internationally are more chronic.
The UK had tried to insulate itself from that possibility so that it didn't lose out for others slow movement.
That is pretty prudent. And was wise in the context of Brexit.
If you reflect on things in that context - contracts which ringfence uk production and required first guaranteed supply (rather than best efforts) to protect against global shortages, the UK (rather than AZ) position is less about 'stealing' anyone elses supply and more about protection from the impact of decision making elsewhere which might be flawed or suboptimal.
It was obvious from the word go that competition for vaccines initially would be huge and that had the potential to spark a trade war (brexit was always risking trade wars) so thinking about that should have top of any national or international strategy.
The uk was well aware of how exporting vaccine to other countries would look domestically. The EU's strategy on this has been different for various reasons. They are realising that slow ordering, poorer contracts, logistics with Pfizer compared to AZ and production delays are causing problems and are now trying to mitigate problems relating to that. It hasnt properly identified problems with vaccine shortages in the way it should have during planning.
Its difficult to get away from this reality. And the contrast between the uk and eu only makes it look worse. Which is one reason why they aren't insisting on az vaccine coming from india...