'Cabbage' White Butterflies

I'm not absolutely certain whether these are small whites or large whites. There were five hovering around together and drinking water and picking up minerals from some wet earth on the very hot allotment.

The butterflies are fine except that they lay eggs on any brassicas they find and the caterpillars will decimate a plant (or patch) - ours are all under netting now as our sprouts succumbed to this in the first year of having an allotment plot.