2 issues at play here, firstly the second doses have to follow the first at 11-12 weeks, and of course, know how many are coming up. Generally it is about 450k per day Wed-Sat but lower (around 350k/day) Sun-Tue
Don’t know max capacity but it is around 600k (again with less early in week to allow staff days off), so tend to get 90-140k second doses.
Generally the remaining 22% (no doses yet) are under 40, so won’t be getting AZ, so becomes a supply question of the other types.
However the logic that once first round ends, there will not be two parallel lines is true, so the spare capacity (to give) means the 12 week gap should gradually reduce as long as doses are available.
Finally I believe there is a plan to lower the age from 18 to about 14 (age still to be decided), and whilst in size and stature this group may be similar so should be ok, the legal and regulatory process for minors is different, and not currently authorised in UK (but some other countries like Canada now including this group)