The UK could have done what other European, international hub countries and cities did. Borders are still open, but there is a strict testing and quarantine regime everyone adheres to. Why? Because the authorities monitor each returning traveller and make sure they're staying inside if required.
I live in one of those countries and since early summer, negative tests and quarantines have been compulsory when coming back from a risk area. You are spot checked during your isolation. You need to email your negative results to the local health service at the end of the 5/10 days, then you're released from quarantine. There is no faffing about and you don't risk breaking isolation because it's up to €25k in fines, and you do get caught because they are monitoring you. Result - very few imported cases.
I can't believe how almost a year on, the UK has absolutely no system. They're only now considering asking for a negative test on entry?? 10 months on??? How do they plan to monitor those negative tests and the quarantines that follow? Because a negative test on entry doesn't mean you aren't incubating the virus.
Yes, it's probably unrealistic to go full Australia mode and ask people to isolate at the cost of £1k per person. But they should have definitely come up with a system which ensures people are staying in when they need to. No going to the supermarket, or leaving the house for a walk early morning, or having people around during your isolation. If people were monitored and scared of getting a huge fine (or infecting others, that's worse than a fine!) this wouldn't happen.
It's doable, but the UK government decided to not even attempt it. And people feel too special and always have a reason why the rules don't apply to them.