Free to Be an Idiot

A group of people are to be prosecuted for butning an effigy of Grenfell Tower in the privacy of their circle of idiotic friends.
I thought the UK had freedom of speech, which implies the right to be an idiot, if noone else if harmed – even if a lot of people are offended by their having posted a video on the internet.
They should have opted for a full court case. I think they would have won.