Pick them off, search for eggs and destroy them and then cover the plants with some kind of barrier to prevent butterflies laying eggs. Netting or fleece are what's usually used.

Hi Steragram, thanks for your reply,
slaphead

Yup next year net your brassicas or accept the blighters!

I have just put a second lot of fruit netting over mine,
seemed to keep them out.

Wasps! Be kind to wasps, let them nest in your garden/plot. I once watched several of them carry away dozens of caterpillars picked off brassicas.
Pick a size of netting that stops butterflies and large moths but allows through wasps as there are other small species that won't be easily stopped.

A chap on our allotment field did this. Was fascinated by
his Queen Wasp. Then she decided to fly over to my plot
and nest in a hole.
I bashed the hole with my spade. Got badly stung.
It cost the Assn. £40 for the Pest Control Officer to get
rid of them.
The new small guage netting advised by another Goyer will be on my shopping list for next year.