Funding! All the sexual health services in England are seriously underfunded - as part of the Tories' changes to who does the commissioning of health services, since April 2013 the money for STI clinics now comes from local councils. Their financial position means that most of them have reduced services, and many of them are cutting still more this year. On the one hand, doing it by council area is better than letting GPs do it (as they do for much else in England now) but many people have always travelled for their STI testing, especially in London where a handful of central London clinics basically did most of the work.
As an example of what's happened, in many places, the primary service is 'you get sent a testing kit by post'. Just looking at London, it's postcode lottery time: Hillingdon, Hounslow, Sutton, Croydon, Greenwich, and Barking & Dagenham don't fund that, so residents there don't qualify.
If having a comprehensive STI testing and treatment service wasn't a statutory requirement, it'd be even worse.
If you have symptoms, obviously turn up. If you don't.. do you really need testing every six weeks? If you think you do, can you pay for a postal test?
If you don't like this, write to your MP and councillors and let them know that you're pissed off about it.