Possession of banned drugs is illegal, being under the influence while driving is illegal
Not sure consuming drugs is illegal, so tests that show someone has taken a drug, but doesn’t discern whether it’s a sufficient amount to impair driving is going to be a great test.
Pretty sure taking drugs IS illegal in the UK. I guess you might have just returned from somewhere where it is legal and still have legal traces in your system.
I get what you're saying, but it's still, you know, illegal.