Usually a pound. The last time I had a pizza ordered, I said "Take an extra pound for yourself", I had given him a twenty pound note. He said something, then laughed and gave me the change, but never took his tip. I was slightly confused and had no clue what he said before moving on to devour the pizza.
They make you pay a delivery charge or incorporate the delivery into the charge. Nobody delivering pizza gets a tip off me and they shouldn't expect one. If I continue to get deliveries from the same place if they don't start giving me stuff free after 2 weeks I'm off somewhere else. £20+ I want a free bottle of soft drink, £30+ I want free delivery, prawn crackers and some squid if it's lying around. Hell, I used to get the local chinese to pick me up cigarettes on their way round (when I smoked)