The new mahonia eurybracteata subsp. ganpinensis 'Soft Caress' looks like a fern, but is an evergreen shrub and has the characteristic yellow flowers of other mahonias. It was Chelsea plant of the year a couple of years back, and I really love it.
Fatsias - common, but fun! Evergreen bamboos are also nice (clump-forming ones if there is an open street policy - you can get lovely coloured cane varieties if you have sun).
Viburnum tinus - evergreen in summer, lovely white flowers all winter (yay!) with black berries.
Erysimum 'bowles mauve' - this jawdropper flowers in my claggy, north-facing garden 12 months a year. Yep, 12 months a year. I think it will probably flower itself to death in the next year or so, and I will buy it again in a heartbeat.
Variegated dogwood - lovely white and green leaves in the season, then really vibrant bark the rest of the year provided it's pruned.
Smaller acers for spring and autumn colour, and bark interest.
Christmas box (sarococca confusa) for a shot of amazing scent in winter and evergreen foliage.
Evergreen shrubs - variegated eleagus, bay trees and daphne if you have sun.