Section 139 of the Air Navigation Order 2009:
Drunkenness in aircraft
139.—(1) A person shall not enter any aircraft when drunk, or be drunk in any aircraft.
(2) A person shall not, when acting as a member of the crew of any aircraft or being carried in any aircraft for the purpose of so acting, be under the influence of drink or a drug to such an extent as to impair his capacity so to act.
Basically, would you want someone drunk between you and the nearest exit if smoke started to fill into the fuselage?
My experience is that BA are quite strict on this. I am teetotal, but swallowed at least half a packet of Marks and Spencers Wine Gums during a long delay. My breath must have been not too sweet since I was asked at the gate as to how much I had been drinking.