When people mention the UK isolation rules and get critical as they have changed they forget testing is involved. If some one is positive they remain in isolation. That determines how long they isolate - if they obey the rules. Quoting you as knowing the gov the actual real changes in isolation time may well on average be longer than the number of days they suggest. On the other hand vaccination should speed things up.

Only one way to find out but if people don't report they never will. Infection levels show that 20 to 29 year olds do not report, more so males. That looks to be the case at other ages as well.

Or maybe women spend more time talking to each other rather more close together than they ideally should be. Not the first time they lead on infection levels but the way testing is now they may be more inclined to report.

The testing change is why some wonder if the numbers are meaningful any more. Gov says saving PCR for critical workers. Seems it was struggling to keep up.
NZ - their web pages will have info on what they actually do and probably why some where or the other as well. Au's rules have been misreported lately. NZ - pass and I wont be bothering to look. They have followed different ideas from day 1.China is too as are some others. Someday maybe it will be all looked at in detail to see what the best overall approach is but for instance we could not do what China is currently doing.
So the rule isn't perfect for those that choose to cross but if they follow welsh rules they should still work to them on the other side of the bridge as they are welsh. That is about as stupid as your comment especially as populations are of a certain size spread over an area and the majority wont cross it anyway as the boundary is too far away. How bright of you to spot a hole in the idea, congratulations, keep the good work up.