Their reasoning is that if the only things you can do are China-style lockdown or nothing, then the disease will just resume exponentially growing the second lockdown ends, so it's pointless to do it at all.

While that may be technically right (under some optimistic, unverified assumptions, e.g. no reinfection), it's a false dichotomy. Singapore, Hong Kong, and South Korea aren't under lockdown, they're stopping the spread by just having good testing and case tracking capacity, and good hygiene. And the longer things go on, the better they're getting at doing that. If things are getting out of hand, a temporary lockdown is effective (as it was in China), to get the numbers down while building up this kind of capacity. But lockdown doesn't have to last forever; China has already lifted restrictions in most cities.

The UK seems to be ignoring this, and I don't understand why. I suspect the real logic, as some government officials have outright said, is that a completely failed response would free up money for the NHS by killing off "bed blockers".

The social signalling around it is "prudent caution", in the sense that they are talking slowly and confidently while using big words. This gives some people the gut feeling that this must be the "rational" thing to do, but it's just window dressing. The plan is neither prudent nor cautious; it's recklessly endangering millions of lives. Experts in the UK are horrified; there's an hours-old petition against this with hundreds of academics already signed on [0]. The WHO has condemned it, and half the people I know studying abroad in the UK have fled the country over it.

0: http://maths.qmul.ac.uk/~vnicosia/UK_scientists_statement_on...