From a cursory look, I couldn't see a direct law on legislation.gov.uk.
The best I could see was this document from 2010
http://researchbriefings.files.parli...61/SN02061.pdf
It suggests that most airlines ban this practice and that it would be difficult to turn into law.
The legislation mentioned is this
http://www.legislation.gov.uk/ukpga/2003/19/contents which seems to specifically make clear it relates to boarding a plane while drunk.
So, I don't know. It's just a cursory look but initial signs suggest it's on the airline.
Also note, I'm not a legal expert. I think we have some here who will weigh in shortly and tell me I'm wrong
