It will only "cost a pound to spend a penny" if you need to spend a penny... I'm pretty sure that if you don't take advantage of the airport bars (or the in-flight refreshments) before you fly, you can manage a couple of hours without needing to go. Or go before you board...

Or you can gorge on red meat, eggs and bananas the week before you go, then take a laxative before you board just so you can get your full pound's worth whilst in the air... just let me know if you're on the same flight as me so I can give it 10 mins, or open the window!

LTS