I agree, the UK Police wouldn't typically let you get away with "it's just a joke". This would constitute a mixture of identity theft, fake ID and misuse of computers.
I’m not suggesting that everyone most do self-immolation, but if it were me, I would draw the line at being afraid of being “caught” for an obvious prank using no PII. Screw that, come arrest me if things have really gotten that bad.
We hold that LLMs are incapable of generating copyrighted images, so it's not just a tool - if it was just a tool then the author would be able to copyright the images. The courts recognise that an LLM is capable of generating things in its own right (which is why they're not copyrightable - copyrights only protect human works).
So it follows that an LLM must be able to create images itself, separate from the human prompter.
Whether that's enough to absolve the human of the crime, though - IANAL, and I suspect it would take the House of Lords to rule on it definitively.
Whether that's actually legal in this case I don't know, but I'm pretty sure courts won't conclude "welp, it was the AI, not the user" in a case like this.
There isn't one.