From what I understand on 22nd September SAGE presented some alarming modelling which in the following days became available to all parts of the UK. Wales, ultimately, based on that evidence went for a short very restrictive lockdown. England, via the UK government decided not to at that time and ridiculed the need before succumbing a few weeks later and going for a longer later set of measures because "no one could have forseen how malignant the virus was".
Like you say the proof will be in the pudding.