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Bubba is right here,on the buy screen the 2 arrows (1 points up,1 points down) just to the right of the amount of GBP/EUR/BTC you want to enter.
I use my phone so I tap on the arrows to change from GBP >btc,if on a computer click on it with the mouse/touchpad n you'll be able to enter exact BTC amount no probs.
Hope this works for you :-)
Edit :- Quoted wrong name then posted under bubba when I should have posted under gravity...stoner problems lol