I think if they had their time over they'd definitely do things differently. However, mRNA is a new technology and they probably had advice that it was less likely to produce a viable vaccine and that even if it did it would be much more expensive to order and much more challenging to store and distribute. Needing to store in very cold fridges does make it much more difficult to roll-out.
Pfizer have changed things to make it easier to distribute now than the original recommendations, making it possible to distribute through the crucially important GP channel, not just limited mass vaccination hubs.
Had the UoQ candidate been successful and the AZ not been slammed over very rare side effects, you can be sure the Opposition would have been very critical now of the government "wasting money" purchasing 40 million Pfizer doses had they done so when they ordered the original 10 million.