The best thing you can do is to look in your local Garden Centre and see what's in flower now. Then plan ahead and buy seeds, tubers, corms or plants early next year. You could buy a few pots now for some 'instant' colour. I suggest Dahlias and perennial Chrysanthemums. The Dahlia tubers should be lifted when the first frosts arrive, and the tubers stored in a frost-free dry place until next year. The Chryanths can stay put.

I can tell you what's in flower in my garden..well - some of them!

Phlox paniculata
Heleniums
Penstemon
Coreopsis
Lysimachia ephemerum and L. fortunei
Dahlias
Crocosmia
Ligularia
Clematis
Roses
Verbena bonariensis and V. rigida
Asters (perennials)
Rudbeckias
Echinaceas
Salvias.
Chrysanthemums
Lythrum salicaria