[londons_explore]

Making Change for a £50 really isn't hard - it's 2 20's and a 10. Those notes that the cash register is probably already full of...

[Symbiote]

This is exactly right: you can easily accept £50 notes, because you almost certainly already have some £20 notes which have no purpose — other than making change for purchases of ≤£30 with a £50.