In the UK the best place to turn to for a credit report is Experian. Their website for individuals is creditscore.co.uk. The services offered are:
They offer a 30-day free trial for membership, and after signing up to that you can pay a small amount (£6 I think) to get your credit score. You need to provide various bits of information to verify your identity. The credit score is the main thing that lending institutions go by when assessing your credit risk. You can also view your full credit report which is effectively the data that are used to compute the score.
If you want to cancel your trial membership, be warned: it takes a (freephone) phone call, and the person you speak to will almost certainly point something out to you that is wrong with your report or personal details. In my case my previous addresses weren't listed in my profile (although they did appear in my credit report) so I decided not to cancel the membership at the time. They will also suggest that you might like to carry on using the service for keeping an eye on your report, in case of identity theft for example. It's up to you, but I found that a one-off check on the free-trial membership was sufficient.
Bear in mind that you may not discover what's wrong. Both Julii and I checked our credit scores because of a failed joint account application, and it turned out that our scores were 984 and 999 respectively, ie, extremely high. Interestingly neither of our reports showed that we were associated with each other, despite an existing joint account (not listed on my credit report) and joint credit cards. The best explanation that the Experian employee could offer was that the institution that failed us (Norwich & Peterborough Building Society) simply had its own criteria that were unrelated to our credit scores.