There was and still may be, an element within the CAA medical section, that advocated head hair testing for drug and alcohol use and abuse, especially for cabin crew. It's fairly common knowledge, for example, that blood gamma levels often return to normal after six weeks alcohol abstinence even in the heaviest of drinkers. Thereafter, one can donate a blood specimen and return to heavy drinking and repeat this cycle until death or an accident occurs. This would not be the case with a hair sample although that test itself is open to its own controversy.
By some imp of the perverse but very British in its thought logic and in that country, a hair test that proves positive might be used as prove of alcohol abuse while one that was negative may not be used as evidence of innocence in excessive imbibing.