Euphorbia corollata

A white flowered Euphorbia - who knew? In this case fairly normal looking bushy spurge plants produce increasingly wide branching sprays of small pure white flowers in summer – more like a Gypsophila or a Linum perhaps.

Completely untried here – the species originates in Eastern North America and should be completely cold hardy and easy in a sunny spot.
3L pots ~ £8

Seseli montanum and/or hippomarathrum

As with the Athamantas I offered before, I'm not totally sure of the id of this. I seem to have had the same thing under two names - S.montanum and S.hippomarathrum. Either way it's a lovely small umbellifer with a compact cluster of finely divided green foliage at the base but fairly tall flowering stems. 

The buds are reddish, opening white. Easy in any well drained sunny spot. Long lived but seeds about mildly and easy to control.

3in pots ~ £5

Carex baccans

A typical tussock-forming sedge in many ways except that (surprise surprise!) it makes shiny red berry-like fruits in autumn instead of the usual greenish seeds. Unaccountably hard to come by.

Not a great shot, but you get the idea. It probably needs more space to make a nice big tussock.
Cool and moist woodsy conditions are recommended. 

1L pots ~ £8