jordielaz
If you just want the law on this matter, and not a debate, why didn't you just google the topic?
You would have found information like this:
http://stop-racist-human-skin-phobia.org/laws. html
And this
Contrary to popular belief, there is no law in Britain that makes public nudity illegal. Victorian laws on �indecent exposure� were repealed a few years ago, and were replaced by a law that makes exposure an offence only if it is intended to cause alarm or distress. A government minister confirmed that this law does not affect a genuine naked walker who has no such intention. Provided walkers avoid built-up areas and crowded places when walking without clothes, there is little risk of committing an offence."http://www.walkingclub.org.uk/walking/naked_walking.shtml
The man has not committed any offence by wearing his underpants in public view.