I'm not British, but I love Derren Brown as much as I love Sherlock. (Just discovered him years ago on YT.) I'm not sure, but I think he's really well known in Britain, his live Russion roulette stunt, The Heist and predicting the lottery were shows that made people talk. So he's very much part of the British celebrity culture.
Moreover, he fits really well in the show and its culture. If you ever watch his own shows you'd notice a similarity in tone with Sherlock, the look, the affinity for horror, the misshief, the mind control. He's also part of the sceptic tradition amongst magicians that started with Houdini, Doyle's close friend, and one of the most well known sceptic magicians in Britain. Apparantly, he and Mark are friends (and if they aren't, they should be, the seem to be similar types) and Mark has co-written one of his shows, the one with zombies.
-rant over-