I am concerned about cutting budliea incase its useful to butterfly eggs .
Not entirely sure but I'm fairly confident that butterfly's and other pollinators are only interested in the nectar the flowers produce and butterfly's and other insects have no or very little interest in the rest of the plant.
No idea though if now is the right time to prune buddleia back.
Butterflies feed on buddleia but the caterpillars don't. If you dead head at this time of year, they will often reflower from side shoots. If it has finished flowering, cut down by a third to stop root rock in winter. In March, cut it down to about 30 cm or so.
I am assuming you have Buddleja davidii hybrids, not globosa or weyerana or alternifolia. They are pruned differently.