Large & Small Whites
Commonly known as cabbage whites, and umbrella term for both Large and Small Whites - and they can quickly reduce cabbade leaves to a skeleton. As you have probably already guessed, the Large is bigger than the Small White, by up to 10mm (1cm).
Other differences between types are:
The black tip - reaches at least halway down the edge of the forewing on Large. In Small it can be black or greay, and extends further along the top of the wing than down the outer edge.
If you see a White out in March, it is most likely a Small. Large don't tend to emerge until a good month later.
Both are common in gardens and parks, as well as open fields, and can be spotted up until October. If you see two dancing around each other in flight, they are doing their mating flight.
WILD CARD: It's easy to tell the males and females apart in both Large and Small. Males have only one black or grey spot on their forewings, but females have two. Both sexes have two spots on the underside of their wings.
Main picture: Three Small Whites feed. The one on the left is male.
Above: A Large White female. Because she is relatively old her wings are getting faded and tattered, so the black tip can no longer be so clearly seen.
Right: Only in certain light can the underwing of the Large White be seen to be a soft green/yellow. Note the subtle slash of brighter yellow across the wing. Also note the hint of black running along the top of the forewing, which can just be made, and which gives this butterfly away as a Large White.
Left: The Marbled White has dusty-grey markings on its underwing, and a slower flight than the Large White.