I'm sorry, I feel bad about that last one...
Have you checked your credit rating? There's probably some adverse information on you - maybe because you haven't made payments on time in the past, or you've defaulted on a loan. Or maybe you've moved house recently and are not yet on the electoral roll?
I believe that the top companies in the UK for credit checks are Experian and Equifax - there's another biggy, but I can't think of it off the top of my head (someone else on AB will know). You could contact these agencies for your credit record and, if there's any information that doesn't seem right, you can apply to have it amended.
Having said this, the credit-check companies provide information, but the final decision as to whether or not to provide credit is made by the individual company to which you're applying.