As I see it, there is no clear identified advantage to mixing AZ/PZ for the booster at this stage. However, given that the Government is only offering PZ for boosters, then for those who had AZ first two times around and qualify for a booster, it will offer a good local test case somewhere down the track.
As for me, unless there is a recognised study that propounds a clear increase in protection from such a mix, I am likely to have a third AZ shot in late January, about 5-6 weeks before leaving for the UK. Having tolerated the AZ vaccine well the first two times around I am happy to go with what I know works for me. I don't see the need or benefit to mix it up with both AZ & Pfizer in the hope that it gives me a significant boost in protection if in fact benefits are marginal at best.
We seem to have settled the vaccine certificate issue in that the third dose will show, but is not necessary to be recognised as fully vaccinated either at home or abroad. TTFN