Most GP's can do this and run a drug test (for a fee of course). It involves a blood draw and there'll be a delay of around 5 working days IIRC
If there is a test you wish to do for something specific (as against the standard "recreationals" of alcohol, benzodiadepines, opiates, cocaine / amphets, etc) you'll need to inform the GP.
You'll also need to tell the GP about meds you are taking in case of "false positives" (one drug having chemically similarities to another, but having a different purpose / effect.