The Royal Horticultural Society gardens at Wisley have plenty to see through the seasons including autumn. I paid a visit there a few days ago for their annual plant sale (and to have a look around of course).
 North America is a good source of late flowering perennials including various Eupatorium species which they call Joe Pye Weed over there. Several of these form striking clusters over 6 feet high with red-purple flowers that are much visited by butterflies and bees.
 So they're a bit out of scale for a London back garden but a couple of weeks ago I went to a plant sale in Highgate where Edulis had a stall; they cultivate many interesting and unusual plants from around the world. Among them they have a cultivar of Eupatorium dubium which goes by the name of "Little Joe" that grows to 3 or 4 feet tall only. I couldn't resist buying a few pots of it...