You can indeed do the GBP to Euro conversion using the "currency" option in the edit menu, click it, let the window open and then use the drop down menus at the bottom to add a new option to the original list and then type in the current exchange rate for it, select OK to finish adding the new entry and it should be one of the choices in the lists. (the current conversion rate is 1.46503 euros to the pound.)
Make sure you update the currency rates when you start the calculator to get all the rates loaded up