Every major credit card I have used automatically does the currency conversion for you. Just order as you would and when you get your statement it will be adjusted for the exchange rate at time of purchase. Most cards also charge a small fee for this.
Actual cost will depend on exchange rate. If the item cost 35$ us then it will probably end up costing you 25 pounds or so plus a small fee for the conversions (imposed by your card company). This is a rough estimate. There are tons of currency converters online xe.com is one I think. But the cc company will handle all that. It will automatically convert at the correct exchange rate.