Aster x frikartii 'Monch':
Soft lavender blue flowers in late summer. Very early flowering and shorter than many asters at about 80cm. Easy and reliable.
Calamintha nepeta 'Blue Cloud':
Spikes of small, soft blue flowers above mounds of minty scented green leaves. Loved by bees and butterflies. Hardy. Up to 30cm.
Salvia uliginosa (Bog Sage):
Branched stems of clear blue flowers and lance-shaped leaves. Needs a sunny spot but with moist soil. Late flowering. To 2m.