I think "the last flowers of summer" will be a theme in the next week or two, if that isn't too melancholy. Actually I love autumn but it's true that wildflowers are becoming scarce by this point in the year.

 Harebell (Campanula rotundifolia) flowers from July into September and can be found on dry grassland and heaths. I've seen them a lot in areas like the Pegsden Hills in Hertfordshire recently but they can be found not too far from my flat in London. Blackheath is a fragment of what would have been countryside in centuries past and some of the flora survives. Harebells look fragile but they are extremely tough and persistent spreading via underground runners.