Is this an arbitrage opportunity?
You also need to consider the exchange rate between the two currencies to determine if this truely is an arbitrage opportunity.
1 British Pound currently buys 1.39 US Dollars1 whereas the transfer via BTC using the figures you have quoted yeilds 1.724823486 times.
This is indeed an arbitrage opportuniy if you can buy BTC in British Pounds and sell them for more US Dollars, provided you have some British Pounds to buy them with initially (and allowing for any additional cost of buying them, and the transaction fees).
If you can instantly buy and instantly sell this is possibly (the disclaimer) a large opportunity.
1. According to Big Internet Search Engine.