How many decks in the shoe in a UK casino? I'd think that they're less concerned about him manipulating or magically changing the pips to turn a trey into an ace, but would be very concerned that he'd be an excellent counter.
Kreskin was quite adept at mnemonics and once IDed a hundred or so people, by name, in the Tonight Show audience. Even if it was a trick (this was long before Kreskin started eking over the line to woo), it's the kind of thing people believe mentalists and mnemonists do. Even in an 8 deck shoe, it's probably not that hard for a trained mentalist (trained in mnemonics) to calculate the plus/minus values.
Casinos, rightly or wrongly, can ban anyone they think is counting. In the case of Brown, I'm sure it's a preemptive strike. They're just SURE he's going to do something to take away their edge. (I don't agree with this - I'm just stating the policy. I think that if someone develops a talent for knowing the value of the cards exposed and remaining, they should be allowed to play as long as it's all between the cards and player with no external devices involved. Banning them is sort of like banning a bridge player who has a better memory of the cards that have been played already.)