Easy to grow plants with long seasons (please forgive any mis-spelling!):
Astrantia, shorter autumn flowering "Michaelmas daisies", hellebore, dicentra spectabilis (now has new name), foxgloves, santolina with pale button flowers, Anthemis punctata, herbaceous geraniums especially phaem varieties, phlox, peonies (short flowering season but lovely foliage).
Hayloft plants sell collections of salvia, penstemon, geranium and phlox as plug plants.
I also have a short, healthy, bushy, constantly flowering rose called "White Patio" which is good value. Do steer clear of ground cover roses which spread too much.