B&Q Doing a fair few perennial (they'll come back each year) plants at 5 for £5 instore. Pick of the selection are the Heuchera's.
Grown mostly for foliage, these will keep their leaves and colour for the winter as well and work brilliantly in any planting combination.
Some ideas here: http://www.crocus.co.uk/plants/_/perennials/heuchera-villosa-palace-purple/classid.2968/
Get them in now and by the time Spring comes round they'll be well established and will be at the sort of size that will be setting you back about £6-7 a plant at a garden centre.