Before jumping to the conclusion that the success rate for any given fertility treatment is all you need to know about a clinic, we should stop and take into account what factors could influence the numbers of people who ended up with babies after being treated by a particular clinic. Remember that some clinics specialize in treating older patients, or those with specific complications which make success less likely. Other fertility clinics might assist more single women or lesbian couples get pregnant, to name an example, and have higher success rates because the patients they see are not actually infertile. Because of these subjective factors, judging your clinic by more than just its success rates is quite important. Of course, fertility clinics themselves are likely to present their success rates to you.
But I think that official government sources of information tend to be more reliable than private sources, especially if there is a vested interest in looking as competent as possible. Therefore, if you are interested in checking out what the success rates of a clinic you are considering being treated at are, I recommend you contact the HFEA, the Human Fertility and Embryology Authority. The HFEA is Britain's fertility watchdog an official part of the UK government and they keep track of the success rates of all fertility clinics on British soil.