WNBR reassure potential riders that there are no laws against nudity in Britain and arrest is not a threat- they state:

If you are riding in London, Brighton, Manchester, Southampton, or York, there is almost no risk of arrest. Police are fully aware of our bike protests, the routes and the nudity it involves. The London ride had police approval every year since 2005 and police even provide traffic control.

Most police officers provide support, with some being less friendly, but unthreatening – as creator of the group, Conrad Schmidt, said:

 This grumpy police officer came up to me and told me that he thought that what I was wearing was indecent. I looked him straight in the eye and told him that I thought his opinion was indecent. I was right; and no, I was not arrested.

So basically – it’s highly unlikely you’ll be arrested for stripping down in public. However, organisers do ask that you wear clothes before and after the event, to maintain relationships with authorities.