From the Home Screen, tap the cog icon in the bottom right corner to open the settings page. Then tap the ‘Native currency’ button:
This will bring up a list of currencies from which you can select. In this case, select the British Pound or Euro as your default currency.
When you go to make a payment, tap the 'quick tap' button for the amount you want to send. This will automatically calculate the amount of Bitcoin for you. For this example below, we will hit the £10 button.
As shown below, by tapping the £10 option, it will convert and select the exact amount of BTC to send.
You are now able to swipe right and send the exact amount of BTC that is equivalent to the GBP amount you selected. In this case £10.
Alternatively, if your native currency is set to fiat, you can enter the fiat amount first e.g £22.50 and then change the account by tapping the dropdown and changing to the bitcoin account. This will convert £22.50 worth of bitcoin to send.