I don't know how well this works if you want to buy an ex-US ticket but pay in GBP. However, my experience of buying ex-EU tickets but paying in GBP is that the GBP price quote given by ITA on the same day is exactly the GBP amount which BA charges, correct to the penny (before the offline fee and UK credit card fee are added, of course). I suspect that this is because a common set of exchange rates is being used by the industry. So this might be a workaround for you if the ba.com "Currency calculator" link on the firm price quote page isn't close enough for your purposes.