choosing a plant

You could try a potentilla - I think they need at least a half day of sun, most do better with full sun. They are very long flowering and come in a variety of flower colours and sizes. Potentill Gibson's Scarlet is one that can cope with some shade and flowers June - August. It is about 15 inches high. Crocus.co.uk sell them. There are a lot of other possible varieties with different flower colours. Some varieties are a lot larger than you want though.

Thrift - Armeria maritima, grows 20cm high and 30cm spread and loves full sun and it comes with white flowers or pink flowers whichever of those colours you may like, google it to have a look.

penstemons, once they start keep going into early autumn and they come in a range of colours, white, red, pink, purple, mauve, blue. yes i do like them.
monarda's or bees balm or bergamot will also fit the bill.