Hello.
The white butterflies are using your lavender to feed from (the
nectar) and are using your brassicas to lay their eggs.
Do go ahead with your lavender hedge because I think you will find
that a great number of different butterflies and other insects will
feed from it. My lavenders are currently being used by several
varieties of butterfly to feed from including blues. All types of bees
love it too.
Hope this is of some help,
Helen.