The easiest way to get hold of cash is from the 24hr ATMs that can be found all over the city.
Hotels One pound per bag is standard; gratuity for cleaning staff is completely at your discretion.
Pubs Not expected unless table service is provided, then £1 for a round of drinks.
Restaurants For decent service 10% and up to 15% at more expensive places. Check to see if service has been added to the bill already (most likely for large groups).
Taxis Generally rounded up to nearest pound.