Have the rules changed in recent months? I've just watched Grant Shapps on BBC1 saying that airlines have the responsibility to check paperwork prior to departure into the UK (passenger locator form). Over the summer I flew KLM, easyJet and Wizz on six separate occasions and not once was my form checked either at the departure airport nor arrival airport. If it is a rule change, it is a welcome one, but I didn't see any evidence of this being correct earlier this year.
And I remember on one occasion at LHR T2 spending 45 minutes queueing at immigration whilst no one from overseas visiting the UK had any understanding of this form (and many UK citizens for that matter).
Add to that and a 80% failure rate of the self isolation rules.