[KaiserPro]

> Exponential growth in positive tests doesn't imply infinite (limited only by the size of the population itself) exponential growth of severe cases or deaths.

Correct!

but when in the growth phase we can't know when it will stop. We have a fixed[1] upper bound on the number of people we can deal with at one go. so when we see we are getting close, we have to take drastic action.

We know that testing in the uk is a proxy for actual infection, it tends to favour symptomatic as its "self selecting". The ONS survey is more accurate but has a significant lag.

I really hope that you are correct about asymptomatic. but we can't be sure, yet.