The question is why...I think that is the most interesting part, not what has been said, but what has been left unsaid.
Well, my two eurocents worth:
Many people have been clamouring for the Borders to be shut for twelve months. Fortunately even the current bunch in the cabinet knew economically and logistically that's a non starter for the UK so they held the line at mainly domestic restrictions and temporary bans on foreign travel combined with quarantine at home (neither of which appear to have been policed or enforced in the way similar rules were enforced in other countries, )...
Several months down the road the new variants appear... plus there's continued grumbling on Social Media and in the MSM from those who are convinced everybody arriving in the UK is, or has been, on holiday, or even worse, is an "influencer".
Ministers look at the numbers coming through airports now and realise that the level of arrivals has dropped to the point where the government can appeal to some of the voters by appearing to be playing the hard man/hard woman, and do it on the cheap - there are so few passengers arriving from high risk countries they can now bang them up without having to requisition all the hotel rooms in south east England.
Even better better still from a PR POV they can chuck threats of prison terms into the mix.
Ultimately I'm sure there will be sound medical reasons underlying all this to explain the "why" but the politicians love of optics, coupled with the reducing costs of implementing stronger quarantine now, will have played into the "when".