UK value-added tax is 20%. $499 * 1,20 = $598.8, which is about £478. Selling it for £499 would have more than covered the tax difference at current exchange rate, so where do the other £100 come from?
As was pointed out earlier in other comments, Google generally uses a 1:1 convention for USD->GBP for pricing. (This 1:1 generally accounts for minor f/x fluctuations and for customs duties for importing goods into the U.K.)
GBP 500 x 120% = GBP 600 (rounded)