Except actual experts and scientists told them to lock down more than they had because this was a bad strain and they ignored it, so the idea that somehow they didn't know it would be so bad is fucking bullshit. They were told, this is the same shit they pulled when covid hit the UK early, just it isn't that bad and push for letting everyone catch it and be done and then suddenly they realised it was fucking bad and acted like they had no idea. The tories are evil fucks and don't deserve any benefit of the doubt from anyone.
I'm not sure it's that simple, although I would certainly be interested in sources that say otherwise.
Experts wanted the government to act because infection rates were so high, but that was not necessarily because they knew that the rates were so high on account of the new strain of the virus. In any case, the Tories certainly ignored that advice.
IIRC, observers in the Netherlands are also curious as to the exact cause of the explosion of cases in the UK, as they've noted the virus there but without the same exponential growth (yet).