Thanks for the suggestions all of you. There are a few things there I hadn't thought of, so I'll take a look at the RHS plant finder. I do use that a lot, but can't really find the right search terms for what I'm after.
It's more of a lollipop effect, Zixi, but irises are a good example of the sort of things I like so I'm not too fussy. I used to have some echinops plants, but they disappeared a long time ago and I haven't found them to buy in many places recently - I shall have to have another look.