Bar tabs (in addition to tipping culture) are less common outside of NA. It is however quite common to leave the change from say a £5 note for a £4 beer. This is purely down to the convince (no need to wait for your change) and frictionless nature (the bartender already had your tip) of the transaction.
Write with what? I've yet to see a single restaurant put a pen next to bill. What I do is I leave the cash after card payment. Seen the option on card reader for tip maybe 2-3x in my life.