Earlier quoted context omitted.
The UK price is VAT inclusive. (Advertised prices in the UK must generally include tax.) The US price is pre-tax (since there are several thousand sales tax rates that could apply, based on customer location).
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?
GBP 500 x 120% = GBP 600 (rounded)