In the UK it doesn’t matter if its a photo of a real person or not, porn that depicts a child is illegal.

Tbh I think with the use of AI at this point this might be a pretty good law to have. “It was AI generated” is not a defence. Realistically if you are doing it for yourself no one will find out so there is some kind of argument as to it being harmless, but when you start doing other things with those images such as using them for blackmail the police should be able to use that as sufficient evidence to charge you.