There are many websites that have conversion calculators included, and update themselves according to the most recent rates, so they should always be accurate. These include links such as www.xe.com/ucc/ and finance.yahoo.com/currency-converter/#from=USD;to=EUR;amt=1 although a simple Google search can also bring up many others.
These conversion websites are usually free of charge, so it is not worth paying for a service like this when there are complimentary ones available. Most of these also include other major world currencies such as Euros, Canadian Dollar, Australian Dollar and Japanese Yen, as well as less common ones from around the globe.
You can also go to your nearest Post Office to find out the current conversion rates for British Pounds to US Dollars, although check beforehand to see whether they charge for this service or not. You can also order your currency online or in person at a branch, where they usually have the cheapest rates available compared to supermarkets and banks.
Some banks may also offer the conversion rate service, but again ask before you start whether they do charge, otherwise they may give you a fee at the end. They do tend to be quite expensive as this is where they make the most of their money, so make sure you check all the terms and conditions before you go in.