Because the unconstrained R rate is relatively high, ie this virus is relatively easy to ‘catch’.
Strictly speaking it doesn’t need high vaccine compliance, but the only other option is to let the unvaccinated catch it. If 80% of the adult population take the vaccine, and it is 80% effective, that still leaves over a third of the adult population unprotected, and with all restrictions removed most of them would, quickly. That is enough to cause the NHS issues, I imagine.
As it happens, over 10%* of the population have had the virus, and nearly 30% have had the first shot of the vaccine. Whilst there is some overlap it’s reasonable to sssume that over a quarter of the population now has some protection, including nearly all the most vulnerable.
*officially it’s about 6%, but it’s reasonable to assume many people have had it without realising, or simply didn’t ge5 a test.