Even the most moderate drinker should be aware that it is an offence for pilots, including glider pilots, to fly whilst over the prescribed limit for alcohol. The law is very clear on this, and even as a student (including those coming on trial flights) you must not have consumed:-
ANY alcohol within the eight hours before flying
more than 5 units of alcohol in the 12 hours before your flight. (One unit of alcohol is a half pint of regular beer or lager, a single measure [25ml] of spirits, a small glass of wine, or a small glass of "alcopop" such as Smirnoff Ice or Bacardi Breezer)
a substantial amount of alcohol (or "binged") during the 24 hours before your flight
It is against the law to fly in a glider with an alcohol concentration in your blood exceeding 20mg per 100ml. This is one quarter of the drink / drive limit. One drink will take you over this limit. It is effectively a zero tolerance limit!