Paul, you need to pick up an exemption form from your local pharmacy, fill it in and drop it off at the GP practice fro them to complete and send off. The GP has to confirm that you are either currently undergoing treatment for cancer, or are on ongoing treatment for the side effects of cancer or cancer treatment. ED qualifies as a side effect of cancer treatment.
The exemption certificate lasts for 5 years. At the point of renewal, another one will be issued if you still meet the criteria. John is about to apply for his 2nd renewal as, almost 10 years post op, he still needs ED meds.