Reading between the lines of the explanations for what 'smart casual' can and cannot include and taking into account you are talking about London lounges, it seems quite clear that what they are trying to say is we don't want any English football hooligans or lager louts in the lounge.

They are clearly unwelcome given, "This may include themed or fancy dress outfits, clothes with slogans that may cause offense, replica sports kits, beach flip flops, clothing that exposes midriffs or upper thighs, or bare shoulders on men. We also ask that guests do not wear tracksuits while in the lounge – we appreciate that people like to travel comfortably and so welcome guests to change when departing the lounge."

That list would serve as a perfect description of football hooligans and lager louts.