It was a 12 week gap and applied to all vaccines - not just AZ. You are right about the reason for the change from the trial gap.
Earlier this year the NHSCovid app was launched for people to book vaccine appointments online - and there was a fixed 12 week gap between the first and second (and it prevented anyone booking just a first jab - to complete the online appointment booking both dates were needed). Appointments arranged via GPs were a bit more flexible. Now it is 'walk in'.