mitigations available to use will simply control the spread rather than supress
Well that's what mitigation means. Mitigations reduce risks - in the case of SARS-COV-2, they reduce deaths and disablities.
Seat belts, child seats, and speeding limits do not stop all car deaths or injuries. That doesn't mean we should get rid of them. Because they do mitigate, eg., reduce the number.
Also, NZ waited until more of its population was vaccinated. They're well into the booster programme (with high take-up) - and, unlike the UK are doing 5-11 year olds.
And, yes, vaccines do help. No, they're not perfect, no 100% efficacy, and protection can start to wane after time - but like the experts including the WHO say, they do still make a lot of difference.