I saw this at Grosvenor House earlier this year & DID say something to the ahem, gentleman. Basically told him it was a public venue (even if limited to folk on the exec lounge or elites) & his attire was inappropriate, and that he was also giving the rest of us Americans a bad image w/ his belief that he could bounce around in his robe. This wasn't the no-tell, motel for crikey's sake.

Cheers.