How to give a tip without using 'Keep the change' [closed]
I am occasionally in the same situation when I want to tip a taxi driver.
Converting to your problem, I would say:
Make it 4 pounds.
or if that is not clear:
Make it 4 pounds and keep the difference.
This is in my country (Australia) where tipping is uncommon, however.
Edit:
Thinking about it even more, sometimes I add the word "Just" to the front of the sentence. Not sure why.
An equivalent would be: "Just 6£ back is fine."
Sometimes it takes the driver/bartender a moment to realize what you're saying and that you're not trying to cheat them, though.