The Superdrug urine test looks to be the the same type of NAAT test as the GUM clinics use, so it should be just as accurate.
The downside is the cost and the time taken to post the tube to you, and for you to return it, then the delay in getting antibiotics if it's positive. If it's gonorrhoea, the treatment of choice includes an injection so you'll need to go and see someone anyway.
The GUM clinic will do additional tests (including collecting a sample to examine with a microscope) and may well give you antibiotics there and then based on the results. They may also do a blood test for viruses like Hepatitis or HIV.
If you have symptoms, I'd say go to the clinic. If you just want a peace of mind check the kit is probably good enough.