OP,
I hope you are okay by now. I am a bit too late to advise now but I will anyway.
If someone has any symptoms down below, please don't wait for any postal test results. The whole process of receiving the kit and then the results can take even a week and in the meantime it might get more and more painful ( I am not a man but I can imagine it doesn't get better with time ).
Postals are great for people who just want to get themselves routinely tested and don't have time or want to go to GUM and are able to receive them to their address.
If you have any kind of burning/itching/discharge down below then you need to see a doctor right away.
They will take a swab from urethra and after a few minutes tell you what it is as they can see it under microscope I guess. Chlamydia and gonorrhea have different type of treatment so they will start treating you straight away.
If you don't want to go to DSE, there are hospitals across London and outside who have their GUM wing. Homerton Hospital, Leadenhall in the City, Chelsea and Westminster, Hammersmith, St. Mary's, there is one good one in Kingston etc etc. You do a search on website and give them a ring first and since you have symptoms they should be able to see you ASAP.
Covid made things harder in terms of being able to walk in like before, but since you are an urgent case they may still accommodate you straight away. You may have to wait several hours but health is health.
Good luck.