If you’re an international traveller we recommend that you use your business bank account and your business credit card for all business activity. If you really have to use personal cash or a personal credit card for business costs, then you will need to prepare a separate claim form for each currency involved. It therefore makes sense to keep business items business and personal items personal and you wouldn’t need to do this exercise.
If you wish to add your own conversion rates and show the column totals in GBP you can. When we work on theses documents we will use your GBP figures if they are present, and if not we will use the HMRC approved rates given here http://customs.hmrc.gov.uk/channelsPortalWebApp/ and log the details in GBP.
If you ever need to draw cash from foreign ATMs please remember that we will treat that as “drawings” in the same way as we treat cash drawn from UK ATMs. Getting foreign cash either at an airport or from an ATM is not an “expense”, it’s just a conversion of one form of funds into another form. It’s the receipts for expenses which are needed to substantiate any purchase of goods or services.