To be honest I have only tested with USD and EUR. Those 2 options appear if you click the gear icon next to the title of the event calendar pro module.
You can add additional currencies in Administration Menu > Core Data Administration > Currencies
and they will appear in the list.
The important part to get right when adding a currency is the Code, it must be a valid ISO Currency code recognized by .NET. It "should" work with other currencies, but support for different currencies will vary for payment providers like PayPal and google, an Authorize.NET. I'm not sure if they support GBP.