What exactly is wrong with Britain that this wouldn't be strictly illegal anyway?
It's generally not illegal in the US, either. Taking pictures of people in public spaces is presumptively legal because there's no reasonable expectation of privacy. Now, many have made the highly sensible argument that while you may not have a general expectation of privacy in public, you still have a specific expectation of privacy under your clothes. But legally it's still a bit nebulous; courts in states as disparate as Oregon and Texas have held that taking upskirt photos of women (even children) in public is legal. Both of those states now have laws to address that problem -- Oregon passed a law last May and Texas passed a new law to address constitutional concerns with its existing law -- but many states have no relevant laws about this, which makes it difficult to punish.
It's honestly a really interesting area of law to read about. One big reason taking photos of public figures in public places is heavily protected is that photographers -- even and especially paparazzi -- argue that by doing so they're reporting news. And you can construct a pretty good argument to that effect; if public figures could quash embarrassing photos and videos of themselves from being published, we'd lose a lot of newsworthy content. Think Ford falling down the stairs of Air Force One. While I think we would agree that the inside of an actress' dress does not qualify as newsworthy, the issue is that when you start drawing those lines someone has to, well, draw the lines. You're inevitably putting government in the position of deciding what is and is not "legitimate" news, which is why press protections have remained so strong even for people who are just trying to take nudie photos. That's why you really need laws which both address these specific situations and do so in a way that will pass constitutional scrutiny.
Having said all that, I'm not entirely sure that this would be legal in the UK. The UK passed laws significantly restricting paparazzi after the death of Princess Diana. And the UK does often have stronger tort laws to begin with; protection from defamation, for example, is much stronger in the UK than it is in the US. That's why libel tourism exists. Personally I'm not sure that's a good thing -- and many US states have passed laws to invalidate unconstitutional foreign rulings on libel and slander -- but it is what it is.