Well its always been a numbers game for them, and in terms of the big picture its not as serious as it once was, otherwise we'd never have escaped the cycle of lockdowns.
On a whole population level its not as serious these days because the combination of huge number of vaccinations, prior infections and lots of vulnerable people already being dead has changed the population immunity picture. And although studies showed waning protection as time passes since last vaccine, I dont think I've seen any that show things like 'risk of hospitalisation' going all the way back to where it was before anybody was vaccinated. And theres more than one part of the immune system fighting the virus on different levels, so the fading of one sort of immunity doesnt tell the whole waning story. But some forms of immunity get weak in older people, which is why they feel the need to do a spring booster for certain groups in addition to the autumn vaccinations.
Combine that picture with the fact that age has always been very strongly correlated to covid risk of hospitalisation or death, and the usual cold calculations, and thats why they settled on the current limits. Plus some improvements to treatment options and changes to the virus itself which further reduced the burden on intensive care beds especially.
However this isnt the whole story, because on an individual level some people never stopped being at risk from and dying from covid, and the number of hospitalisations can still place an uncomfortable strain on the NHS. Combine that with other winter NHS pressures and the unknowns about quite how much specific new variants can bust past prior immunity, and there is still room for them to miscalculate, or get nervous and change the rules.
Every time there is a new wave there is some uncertainty about the scale of the implications, every time there is a significant new variant there is educated guesswork going on and risks being taken. Absolute maximum caution has never been on the agenda, not even when the pandemic was new and there was no population immunity and no vaccines. And since the population immunity picture is not what it was when the virus first arrived, authorities are far more willing to gamble these days, especially for people under 65. And its impossible to predict if or when this could really blow up in their face, or doom any particular individual to death. They were a bit nervous about the unknowns with the variant that emerged in recent months, due to the number of mutations and the theoretical implication of those mutations, which is why they brought forwards the timing of this vaccine campaign. As they get more data about this variant, they might change other aspects of the current rules about vaccination, or they might decide to carry on without further broadening of the vaccination campaign or introducing other responses, depending on what the new knowledge does to their numbers game predictions.
I'll ask when I go back for a flu jab (which they haven't got in yet).
