I think there should be a higher standard for politicians than just "not illegal". (That also applies to people on the right) But agree I don't think it should be illegal as incitement to violence because it's arguably not serious. It is crass though and should absolutely be judged
In fairness to Jo Brand, she did make the joke as an offhand comment in a show called "Heretics" where comedians are supposed to argue outrageous things and there's arguably a higher tolerance for edgy comedy and dark jokes. It did generate a lot of outrage at the time and if I remember she did have to apologise and the show got cancelled for a while. I think that's quite a lot for comedy. And would allow comedians a much wider margin for edgy comments than politicians because they are completely different roles.