Adding to Paul's explaination, all stock imported into the UK is bought in $$, so the exchange rate also have a massive impact on prices of components on a day to day, week to week basis. In terms of a relatively large drop in price, this normally doesn't happen until a replacement model is launched or the model is discontinued and distributors/retailers want to get rid of stock.
Wow, i assumed you got all of your stock from Britain?
Come now, Britain hasnt manufactured anything more complicated than a toothpick in 50 years.
Most if not all components are manufactured in the far east, who only accept USD as currency, so this is how it affects the price.
Nothing is made in America either