Any question answered...

It's not an offence unless it is "likely to cause harassment, alarm or distress".

Arguably, a small sticker on a bike is unlikely to do so, and the fact that a jobsworth bizzy doesn't like it is neither here nor there.

We're missing some information here: did he deploy that note because he happened to spot the cuntmobile, or because an offended person (possibly on a day trip from Tumbridge Wells) came into the station demanding that something be done about it ?

We don't know.

In the latter case his response seems reasonable to me. He has to do something to get rid of the complainant. Saying "yes sir, it refers to your mum" might have been what he wanted to do but he knew he couldn't.