I'm sure travellers to the United Kingdom carrying an overseas/non-UK debit card can relate to this question.
Most cash machines start dispensing starting from £10 and in multiples of £10. That means if are just about to leave the United Kingdom and you just need £2-3, you will not only have to leave with £7-8 worth of heavy coins, but you also incur the unnecessary non-UK debit card exchange rates and foreign ATM charges for those unwanted £7-8. Are there cash machines in central London / Tube Zone 1 area which can dispense a crisp £5 note?