Which website for credit score is the most accurate?
They are all accurate. Your score is based on a few things that vary between bureaus - the information in the record at that bureau and the algorithm they use. Since not all creditors report to all bureaus, you may have a trade line in one bureau that is not in the other. Second, the formulas used vary between bureaus, As for Credit Karma, they are probably the least accurate since their scores are more of a guess based on whatever they have. I am not sure what the laws are in the UK, but if possible, pull your credit reports and compare them. My initial guess is that you may have an error in one of the reports (either a missing bad trade line at Experian or a mis-reported bad line (such as a collections account) being reported on Clear. Cross reference both scores with Credit Karma just to get a handle on where you might be. And while you are at it, check each line of both credit reports for errors and get them fixed through the dispute resolution process, even if it was only a former address.
Which site does not mean as much as the same site. Credit Karma is my fav. The scores that go to 999 just confuse you. I've never heard of clear score. May have to look that up. Oh, I am on the other side of the pond, no wonder. The wide differences in your scores suggests something is really wrong with one of the scores.
Since Experian goes up to 840, I'd say the Experian one is very, very wrong. Just use Credit Karma. It's not perfect by any means, but their score is at least usually very approximate.