That's all a load of garbage, though, and won't happen.
It'll only tell you to isolate if you've spent a decent amount of time (I forget what it is) close to someone who's tested positive. You should isolate if that happens, grim though it is.
I agree with the latter (or maybe the 80% figure, because that stops people trying to get traced deliberately for a free 2 weeks off). The former wouldn't really work in my view.
Which is what it does when done properly, it delves deep into your life to see where you've been. It is invasive, but it only works if it's invasive. One upside of the app is that it sort of does that for you - that person you were stood next to in a long queue and were a bit close to will be traced, for example, but without any loss of privacy.
That's just how life kind-of is, and it's not just like that in the UK. OK, we probably had an unprecedented period of stuff being quite good from the fall of the Berlin wall up to 9/11, but other than that...