BJCC
Urine gives absolutely no indication of blood alcohol (or blood drug) levels.
None at all. Ask any Doctor.
Urine contains what was in your blood after it has been filtered and metabolised by your body. Urine is stored in the bladder until you can find a convenient opportunity to have a pee.
To all intents and purposes, anything in the bladder has all ready been excreted from the body.
Hence any prosecution based only on a urine test, would mean that you were being charged for failing to take a pee!
(and that is taking the pee!)
Urine testing can only ever demonstrate (historically) that a substance has been taken sometime (perhaps a very long time ago). It should never be used in the context of this legislation.
Whoever framed the legislation made a very, very bad mistake.
Perhaps deliberately so (consider the undefined nature of "drugs").