Technically in the UK you could take off/land on any public footpath (if there is no byelaw restricting such) or public right of way within the bounds of the drone code (away from people/houses etc). You could even take off and land from a roadside if you were in the middle of nowhere and there is no traffic.
There can be some difficulty establishing if there is a byelaw in place (some parks have these). However, if someone approached you could either a) land immediately if safe to do so or b) move off the land you are on and land your drone elsewhere (again within the constraints of the drone code).
Police are unlikely to take a hard line with people unless they (the pilot) are severely unsafe (low altitude/close to people/near an airport/over built up areas etc), abusive to them or just ignore them. Ignoring them would be the worst thing you could do. Usually a friendly word in your ear is all they will do.
Most people I've come across whilst flying have been nothing other than curious.
Oh, I should mention the power of a high-vis jacket/bib. People generally tend to leave you alone (except of course the police) if you wear one of these because you look more 'official'.
Have fun flying!
PS: I'm only giving general guidance here. I am not a lawyer. Fly at your own risk.