The idea that Kaplan tests are not very good indicators of your performance is actually wide-spread, but it's not my personal opinion. I believe that Kaplan tests tend to be more academic, i.e. focus on straightforward math problems. On test day, you'll likely see a lot of word problems too!
Most people score below their real score on Kaplan for a simple reason: the problems they offer appeal to the academic-math-lovers such as myself. On the other hand, someone who has been out of school for a while (like most test takers) will have a hard time with those babies.
In the end, it's up to you if you want to pay 1000 pounds for that test, but IMHO it's not worth it. Keep in mind one thing: just simulating test conditions is not enough by a long shot! While Kaplan math questions are pretty decent, their verbal is... not exactly GMAT-style. The only book with questions that come close to the official ones was the Powerscore CR Bible. But if you believe that you'd benefit from attending their classes, go ahead! I personally wouldn't spend that much money on GMAT preparation....
