The butterflies that I have seen in my garden during May are:
Brimstone, Comma, Green-veined White, Holly Blue, Large White, Orange Tip, Small White, Speckled Wood
Other common garden butterflies that you might see are:
Common Blue, Painted Lady, Peacock, Red Admiral, Small Tortoiseshell
and more butterflies that might visit your garden in May are:
Brown Argus, Clouded Yellow, Large Skipper, Small Copper
The butterfly-attracting plants that I've seen butterflies visiting in May are:
Aubrieta, Chives and other Alliums, Choisya, Forget-me-not, Honesty, Red Valerian and Virginian Stock
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.
I've also produced an overview of the butterfly year and there's a list of butterflies flying each month on my calendar page.