From a sample of two, Mrs LHRSFO and me, I got the Pfizer jab last week with a 10 week gap before the booster, and she got the AZ jab this week with an 11 week gap. I'm guessing that HMG is giving itself some wiggle room on second jabs in case of supply issues impacting second jabs - and is perhaps anticipating more potential supply issues with Pfizer.
I can see that the second jabs will be more complicated, ensuring that there is sufficient supply, plus anticipating that there will be a higher proportion missing their second jabs (either because they think one is enough, or because they need to rearrange or because the whole thing will seem less urgent then.