OK, a little Googling gives us the public order act of 1986, which suggests that going topless, or even nude, is not in itself illegal in the UK.
It only becomes illegal if the state of undress is used to 'harass, alarm or distress' others.
Essentially this seems to mean you can go topless, or nude, if you want to but if someone asks you to cover up, failing to do so would be a breach of the act.
Ask the naked rambler, arrested 14 times on his Land's End - John O'Groats walk in 2003-4
(Not bother by it myself, but I won't be doing it. Don't want to scare the horses)