Why? That surely defeats the object of the exercise, which is to show that the half/full dose regime works for all age groups.
It will take time to prove the half/full dose might be the most effective. In the meantime, the UK might have to rely on vaccines from Pfizer and Moderna only and there isn't sufficient availability. So, protect the under 55s ( a high percentage of the population) with available AZN vaccine and use scarcer US vaccines on the group 1, group 2 high risk groups.
Ideally, in x months, AZN will have gathered more trial results. There's no guarantee that the half/full dose is best.
Still, AZN and Oxford are a set of duckeggs especially when they knew mid year that there was a trial problem.
They were given instructions on how to set out the trials and they seem to have deviated and not followed them....double duckeggs!