Hi Leslie,
You would need separate budget profiles to cater to your specific requirement. Is the GBP and USD being used in the same Project Structure?? Say one WBS is working for GBP and other one in USD?
If yes then user has to be really mindful in choosing the right budget profile. I am not sure how this will work in real time scenario.. You may need to prototype this in your DEV system and see the system behaviour before proposing this solution.. 
As mentioned by Paul, it is better to streamline the business based on company codes.. then currency and create projects for one company code with budget limits for that company instead of currencies..
Regards
Sammar