Couple of reasons:
1. In the US, there's no VAT (only sales tax, which many states don't have too). UK and most of EU have around 20% of VAT so the price must be at least 20% higher.
2. Shipment cost, advertisement cost, server maintenance cost (for digital edition) are higher in the UK than in the US so the price of game goes even higher. For digital edition, one might argue that there's no shipment cost. However, publishers cannot sell games with lower price because it would put them at odd with the local retailers. Basically if they price the game lower, most people will stop buying the game from stores and that does not good for a retail-publisher's relationship. Also, it's not a good business model too to make different prices for the same products, regardless they are digitally or physically bought.
3. It's not only happen to game industry though. The same product that are made locally are sold at higher price in the EU/UK than in the US. The example would be the price of McDonald. If a hamburger cost 2$ then it would cost around 3-4$ in the EU/UK.
Oh and this:
http://en.wikipedia.org/wiki/Price_index