Well The Guardian's Steve Boxer set out to answer that question. He interviewed SCE UK managing director, Ray Maguire (you had me at hello?), who broke it down into nice easy nuggets that us unwashed masses can understand.
First, there's the VAT, which is 17.5%. As Boxer points out, however, using the US numbers, that would bring the cost of the PS3 up to 352 pounds, which is still 73 short of the 425 pound retail price.
Hmm? What would cost Sony the extra 73 pounds per console? Well, there's the whole exchange rate thing, which due to a very weak dollar, Sony actually has a leg to stand on and Maguire should have just shut up there.
However, Maguire continues and says that the cost of living in the UK is higher, therefore Sony has to pay its employees more. Oh, boo hoo. Cry me a river. So you're screwing the consumer to pay for employees' salaries? Next, he contends that the cost of delivering them to stores is greater than elsewhere and that UK shops expect to make more per console than the rest of the world. Great, now he's blaming the stores that carry the PS3.
So, it seems, that the PS3 is so expensive in the UK because of the VAT, a weak dollar (and your ignorance in comparing the UK price to the US price), the cost of living in the UK, and the stores that carry the PS3.
Somebody here can't accept any blame. Dr. Phil. Oprah. It's time you had a sit down with Sony.