Have I done my bit for the environment?

Leaving pests to flourish does nothing whatever for the environment. When you let caterpillars ruin your greens, you merely help to increase an insect population. Apart from pollination of other flowers by adult butterflies, I can see little benefit. But I do understand your wish to help out nature. Perhaps a more effective way is to develop a natural balance in the garden.
Protect brassicas – perhaps by picking off caterpillars – but at the same time, provide nectar-rich flowers for beneficial insects. It makes sense to control pests with great care as we don’t want to hurt non-target species. Use of protective netting or prudent use of pesticides gives you control without causing environmental damage.