Symphyotrichum novae-angliae (syn. Aster novae-angliae), or Symphyotrichum novi-belgii (syn. Aster novi-belgii), or a close relative of these.
Not one I'm familiar with - which may be significant, as it isn't recorded as naturalised in Britain (unlike the other two I mentioned). But it's quite possible that it is naturalised here, and just overlooked / misidentified.