According to the Universal Currency Converter, the Sterling to Dollar exchange rate at the moment is 1 GBP = 1.74676 USD.
iTunes UK sells singles for £0.79, the US store sells them for $0.99. This means that singles bought from the UK store cost $1.38, making them 39% more expensive.
Music videos from the UK store sell for a massive £1.89 ($3.30), compared to $1.99 which Apple charge for videos on the US store. i.e. UK customers are charged 66% more for videos.
If the rates were the same (according to UCC) the cost in the UK for singles and videos would be £0.57 and £1.14 respectively.