In order to take payments from a foreign country, you would need to have a processor set up for that country. Using America as an example, you would need to have a processor set up for US Dollars, so that if an American donor donated $1, then $1 would be charged. However, if you receive primarily donations form the UK, and your database is set as this, then even with an American processor, the form would show the "£" symbol. In order to show the "$" symbol, you would need to set the database to US Dollars, go live with your Online Form, then switch your database back to GBP within Management > My Organization > Preferences. If you were to reselect "Go Live", it will update the form to show the currency set up in Management.
If a US donor donated $1, and your processor (BBMS) is set to take pounds, the processor would charge the donor one pound, which would show on the donors bank statement as $1.72. The currency you set up in Management, therefore only controls what appears on the form. Therefore, you could set up a form which takes UK donations and then add a link to a conversion site to allow US donors to convert their $1 donation to the corresponding amount in pounds (£0.58 I believe).
With the use of the API module in eTapestry and a third-party web developer, it is possible to create one form which takes two different currencies as donations. For example, one form with a drop down menu that allows the donor to select the currency they wish.
Please contact the appropriate software vendor or IT professional for assistance with this process or issue, which is beyond Blackbaud's scope of support.