Asters come in the shades you want, and should be in flower at the right time, have a look at the china asters, both single and double flowered, Derek.

the problem with real wild flowers is that they wilt [terribly in most cases] once picked and out of water.It may be better to mingle a few with the old tried and trusted wedding flowers just to give a bit of backbone if you know what I mean.

I cant think of many wildflowers/british natives in flower at this time of year.
thing is it sounds as if you dont have flowers on your allotment so to get decent plants you may end up spending more money on plants than you would on flowers.

good plants: Jap anenomes, asters, spray chrysanths might still have cosmos in flower. seed heads from poppies/nigella will also look pretty too.

Sorry to cast a doubt but with last years awful weather it could be a bit risky to rely soley on home grown flowers, by september last year most things, including me.....had given up. :0(