How cold does it get where you live? The choices are few in the colder areas of the UK.

Yep, area's critical - if you're in the south or west, there's quite a few, but up north, well that's a different story.

Libertia has 12" (or more) swordlike leaves and pretty white flowers. Sisyrinchium are about 3" or 4" with blue flowers.

Yucca flaccida Golden Sword or Yucca Garlands Gold are fully hardy and evergreen

. . . should have added that Libertia is evergreen, but I'm not sure about Sisyrinchium.

I have great faith in Bamboo's advice, but aren't Yucca pretty big for the front of a bed?

My Sisyrinchium aren't evergreen, if that's any help.
Ophiopogon little tabby is evergreen here at least - low growing, pretty green and white striped leaves.

Both the Yucca varieties I mentioned only reach one and a half to two feet, Sheilabub, and both are a joy to look at in winter, with their yellow stripes - other varieties do get pretty big.

I'd add Astelia varieties to the list in milder parts of the country, along with smaller varieties of Phormium.

How do Dietes do in the warmer areas of the UK?

Oooh, I love Dietes Tug, but haven't yet been able to obtain it here (seen it in Oz).

Stera - that Ophiopogon sounds good - and of course there's always the ubiquitous Ophiopogon nigra, but it wouldn't show up very well in a border.

True - it needs a light background or it disappears.

I have found that Sisyrinchium does suffer from dieback and can become unsightly. I grow Libertia which does well along the edge of my drive and looks great with the white flowers in summer. Have a look at the dwarf varieties if height is a problem.

I appreciate the way Libertia self-seeds too, Jimmy, but hadn't realised there were dwarf varieties . . . I must investigate. I have a gold one too (Taupo Blaze) - do you know if that one is hardy??

Thanks everyone, will take a look at your suggestions,we are south midlands.