Innova Media, all depends on what you mean. For instance, PayPal doesn't support multi currency and even if user will be able to chose Pounds, payment provider will convert it to USD. Another question on how it will be implement on your site. Will it depend on site language when user change language the currency will change too ? Or it will depend on user country IP ?
As you can see there a lot of options on how to implement the multi currencies functionality. Any way, there is no such default functionality in our software and you will need to make custom code modifications. But It would be much better to do this via creating a plugin.
If you are not familiar with the code modification or plugin development, it would be better for you to find a developer who can help you. You can do it here:
http://www.oxwall.org/market/specialists.