In Scotland, you can leave tips in cash whether you’re paying the bill with cash or card. You won’t generally get a printed bill to write the tip amount on as you would in the US. You just leave your chosen amount with the payment.
Here’s an example:
Let’s say your bill comes to £63.54 and you want to round it up to £70, a £6.46 tip. But you’re paying in cash and you only have £20 notes, so you leave £80. In this case, you would just tell your server to round it up to £70, and they’ll bring you £10 change.