Euonymus fortunii varieties make tolerant attractive evergreen small shrubs that can easily be trimmed to the size you want. Most hebes are evergreen. Summer flowering heathers need an acid soil but winter Ericas are more tolerant - mine are happy just the acid side of neutral, and flower for several months just round about now. Polyanthus might be useful - they do stay green and flower for a long time when happy. They might need watering through any very dry spells. Run out of inspiration - sure there will be lots more suggestions.

I have a heavy clay border and I find that geraniums are very happy there, although they do die back, they are shooting again already, and flower for many months.

I have a couple of varieties of Ceanothus shrubs in my garden - one north facing, one south facing. Both in clay soil and both doing well. Lovely blue flowers .

Hebe's also seem to thrive here too.

Ground cover plants I'd say Phlox would be a good option, or Vinca Minor if you keep it under control.

there are lovely evergreen viburnums [I think mine is called Eve Price] they do well in my south facing front garden, also photinias [red robin and pink marble] look for fully hardy hebes too and elageanus. Also fatsia japonicas, they look rather tropical but are evergreen and hardy,rock roses [cistus] and berberis 'rose glow'
also choisya [ternata] and if you have room for something thatwill grow tall [although you can prune it down]what about the prunus lusitanica?It's fast growing.
In fact all of the above are fairly vigorous plants and will soon grow up if you feed them in Spring.