Was it a sale? Sounds like the item was on sale on Amazon and the normal price was £50.

Edit: Upon looking at Amazon, for me they show as the same price. http://www.amazon.com/Guild-Wars-Online-Game-Code/dp/B0091XHZW8/ref=sr_1_1?ie=UTF8&qid=1363102019&sr=8-1&keywords=guild+wars+2

The link to the non-mobile version of the store page is here