Id try a plant called a butterfly bush. Cant go wrong, they grow fast, and have tons of flowers. The one at my parents house is always loaded with bugs in the spring and summer.
Unfortunately, butterfly bush is an asian species that's highly invasive in many parts of the world, including much of the US and at least some parts of the UK. Because it seeds so prolifically, and the seeds are wind dispersed, it quickly escapes into wild areas where it ousts the native plants, and thereby native bugs and sometimes birds and other wildlife, too. It can survive dry sun to soggy shade, so it's able to clobber a wide variety of habitats. BB does provide nectar, but only nectar. Even then, only generalist nectar feeders come to it. It's about the same use as a vacuum to most species once you count the entire life cycle.
If you explicitly want butterflies, you need larval hosts more than nectar sources--in any town, flowers for nectar are usually all over the place, but larval hosts are scarce, since so many of them aren't showy. Host plants should be massed--laying females are more attracted to a large patch (2mx2m or bigger) than they are to scattered plants because then the caterpillars will have enough food to grow up.
Anecdotal native vs non-native plant attractiveness: I have a european mock orange (Philadelphus coronarius) that came with the house, and a couple of native mock oranges (P. lewisii) that I planted. Except that the european sometimes blooms a few days earlier, they're almost impossible for me to tell apart--they look the same, smell the same. The european rarely has any activity even during bloom time--mostly some european honeybees and sometimes bumbles. The natives are busy from leafing out to leaf fall. I've spotted at least 10 bee species, 5 syrphid flies, 2 bee flies, several tachinid flies, 5 butterflies, a dozen parasitoid wasps, and the assorted spiders (and birds) that come to feast on everyone else. I'd probably see more if I were a morning person...
