Yes you would be paying interest if you short GBP/USD, because of the difference in interest rates between borrowing GBP and lending out USD. If you were long GBP/USD you would likely earn a small amount of interest.
The Oanda interest calculator can help you work it out:
http://fxtrade.oanda.com/tools/fxcalculators/interest_calculator.shtml
It won't be exactly the same as IB, as they have slightly different interest rates - but it is a good approximation.
The Oanda interest calculator can help you work it out:
http://fxtrade.oanda.com/tools/fxcalculators/interest_calculator.shtml
It won't be exactly the same as IB, as they have slightly different interest rates - but it is a good approximation.