https://www.whatdotheyknow.com/request/arrest_of_mr_darren_b...
This is why the Daily Mail causes rolled eyes (along with Spiked and the rest of the right-wing agitprop).
"A 51-year-old man from Aldershot was arrested on suspicion of sending by public communication network an offensive, indecent, obscene, menacing message or matter."
This is the legal basis for the arrest. Without the retweet, police would not have had authority to turn up to his place of residence - twice - and demand entry. No doubt they preferred Brady voluntarily submit himself for interview at the station, but he refused, which I hope we can all agree is the morally correct position. No one should have police turn up outside their house - TWICE - because of a parody retweet.
Those complaints should have been laughed at and ignored.