Instructions:Hidden Mysteries: Buckingham Palace is a PC or Mac game on Bigfishgames. They usually sell it for £6.99 to non-game club members (£4.99 if you're in their club). With this voucher code, it comes to only £1.20. You can download it right away, once you've bought it.
I quite like these hidden object games, there seem to be loads of them now!