Campanula lactiflora (Milky bellflower):
Clusters of blue flowers on tall stems in mid-summer. Ideal for filling out borders and adding height. Fills out quite quickly too. To 1.5m. Hardy.
Eryngium ebracteatum var. poterioides:
Tall and evergreen with attractive blue-green foliage and wiry stems of red-brown bobble-like flowers, Sanguisorba-like. Hardy. Up to 1.5m.
Leucanthemum x superbum 'Wirral Supreme':
Tall form of Shasta daisy with white flowers and multiple shorter petals around the golden centre, making an attractive frilly effect. Hardy. Up to 1.5m.