Correct me if I'm wrong but further down the thread you stated that you received a ban for drug driving. If that's the case then it's absolutely no surprise that you've been required to provide a urine test to check for drugs.
My GP has told me to expect a drugs test when I'm due for my DVLA medical simply because it's noted in my records that I used drugs in the past. The fact that it was over 20 years ago is irrelevant in the DVLA's eyes. Personally I couldn't give a toss, I haven't touched illegal drugs in decades so it will be an exercise in futility.