If this was me I would do a urine sample now and get it to the docs and tell them your think you have Cystitis, then go to the pharmacy and get the sachets you add to water, their normally just called Cystitis relief and you can get generics from ASDA. The sachets work really quickly and will give you fast relief of symptoms, but you may need antibiotics.
The GU clinic you go to may not test for UTI, you could always ring and ask them.