As at 5 Jul, 34M vaccinated with both doses. Another 11M have had one dose. 14 days to 19 Jul (so those 45M together have some protection: different percentages).
Another 8M adults (O/18) are unvaccinated (some through choice). Of those about (maybe) 2M have previously been infected (including asymptomatic) so have anty bodies.
14M under 18s, of whom maybe 3M have previously been infected (including asymptomatic) so have antibodies.
Leaves 17M still 'unprotected' on 19 Jul (almost exactly 25%).
In progress to herd immunity terms (infection, not severe/hospitalisation), that's about 36M equivalents (so 54% equivalent of UK population). The increased infectiousness of the delta variant means its 'R' number is high and therefore the percentage needed to get to herd immunity is higher than for the Alpha variant. We can only estimate that number but maybe 75%. If the vaccine programme continues at pace (hopefully faster than at present) AND JCVI decide that there is merit in vaccinating the youth, we might make this by September. From this PoV only, natural acquisition of antibodies will help.