In the UK (this guidance from the website of the Crown Prosecution Service, I cannot give you specific laws): In the absence of any sexual context and in relation to nudity where the person has no intention to cause alarm or distress it will normally be appropriate to take no action unless members of the public were actually caused harassment, alarm or distress (as opposed to considering the likelihood of this).
So hurray! We can all run around stark bollock naked! Freedom! Liberty... unless some copper, do-gooder, or other ne'er-do-well decides you're causing them or someone else harrassment, alarm or distress. Yup, we have a public nudity law that basically says "long as everyone likes the way you look, it's cool!"