Lot of "ifs" and "variables" there.
If it's Council or Immigration - nothing.
If it's Police, that's different. You're not in a public place, you're on private property. You can be drunk all you want. If you've consumed illicit substances, likely nothing unless you get a bit stroppy in which case you could be arrested for obstructing Police duty. If you're caught with illicit substances on you then you will be arrested for possession. You might have your details taken and be questioned about what you've taken and where you got it from, i.e. did you get it from inside the parlour/brothel.
There is a grey area around whether a search warrant allows an Officer to search a visitor to the premises - technically it does but there is scope for challenge about reasonable grounds to search a person. I'm not a lawyer and I don't want people thinking they can get away with stuff on a technicality. There are good criminal lawyers and equally good prosecutors who will bat you back and forward with their egos like a cat with a mouse. The best advice I can give people is "you know its against the law, don't take the risk". That's me playing mother hen, frankly I don't care what you do but don't come bitchin' to me when you skin your knees and want a band aid.
Also, if someone is visibly blotto and the brothel is out in the middle of an industrial area - expect an RBT at each end of the street in about... ooh... 45 seconds.