A warrant would not have been needed for this arrest.
Edit: Let me caveat because I fired fast and loose with this reply. This is from an American perspective. I am not a UK lawyer so this could very well be perfectly legal. Though I do view it as "bad" and would oppose the introduction of these norms in America.
This section covers it:
I'm not sure of the specifics as to what "public decency" means in the UK but I believe your implication. I think subsection 6 supports my 'theory' (read: wild accusation)? Seeing as it was posted on the internet and could reasonably be avoided.
Even if that's not so, this seems like a tort and should not be handled by the police. Just my opinion.