The 'Promotions' forum is for the posting of promotional material relating to the British adult entertainment industry, as well as the seeking and commissioning of work by models and producers working in the British adult entertainment industry.
It means you go to a clinic (GUM) and get tested (HIV, Hep B/C, Chlamydia, Gonnoreah) and they give you your results in writing, which is commonly known as a 'cert' (short for certificate)..That's what most of us do anyway..