Like most western countries the UK has a mandatory drugs & alcohol testing program. I have never been tested in the UK nor do I know anyone who has (I got tested in Sweden a couple of times on a SH turnround but that's it). I understand entire crews get selected randomly at base but I have not yet experienced it. Anecdotally I believe a few cabin crew have tested +ve for cocaine and been dismissed on the spot but other than that...... nothing.
The threat here is the secret alcoholic who is a master at hiding their addiction. The only context you might be able to detect a colleague abusing alcohol is when you go for a quiet drink after work. He/she is the one who is drinking heavily, or topping up their mixer with miniatures out of their bag/pocket. You can keep an eye on them... but is it an addiction? Or are they just overdoing it tonight because they are going though a bad patch at home? Most of us have a fundamental human desire to avoid confrontation with others. Do you report them, and open up a whole world of pain for them, possibly unnecessarily? Or do you let it slide? I guess that depends on how serious you perceive their problem to be.
All this is tempered with the fact that you may not see/fly with them for a couple of years so in a few of days it will become somebody else's problem. By the same token, airlines don't want the bad publicity that goes with one of their crew getting busted for drink/drugs. The whole area is a Pandora's Box that nobody is in a hurry to open. That is the reality of our business.