As I understand it, groups 1 - 4 account for most of the deaths, but groups 5 - 9 account for most of the cases where people are hospitalised, but they then recover.
So vaccinating groups 5 - 9 will reduce pressure on the NHS by a considerable amount.
But I don't think we have to wait until everyone in groups 5 - 9 is vaccinated before restrictions can be released.
If you vaccinate between half and two thirds of groups 5 - 9, that should reduce pressure on the NHS to a level where they can cope.
Groups 5-9 cover most hospitalisations by virtue of Groups 5 and 6 covering most of them. I don't believe there are many hospitalisations from Groups 7 - 9, so hospital occupancy would be largely dependent on the vaccination of Groups 1 - 6.