The butterflies that I have seen in my garden during May are:
Other common garden butterflies that you might see are:
and more butterflies that might visit your garden in May are:
The butterfly-attracting plants that I've seen butterflies visiting in May are:
Hesperis Matronalis (Sweet Rocket) is likely to flower this month too.
If you're buying summer bedding plants from the garden centre at the end of May, then why not get some French Marigolds for the butterflies.