I like skimmia as it is slow growing, evergreen sweetly scented and if you have male and female plants you will get berries on the female plant. there are some varieties that are self fertile.

Nandina domestica is attractive all year round, with its leaves various shades of red, bronze and green. White flower bunches in summer followed by red berries.

40 x 40cm - is one of those measurements the depth, or just the measurement of a square pot across the top?

Purple Pixie® Dwarf Weeping Loropetalum

Only grows 1 foot tall with a weeping, cascading habit. It's 'evergreen' (purple in this case). Rich deep purple foliage accented with magenta flowers in spring. Easy care & pest/disease resistant.

Loropetalum is lovely, but not hardy in my experience.

Thanks Sheila - I'm always unsure about the weather differences in the UK.

That’s pretty amazing - and lucky! - if they’re hardy in New York.

No, they won't grow here in NY either, but very popular in the South esp when I'm visiting family in the Carolinas which is temperate. I thought Suffolk UK was in a similar temperate zone.

I think I’d go for a Pieris...small root ball, evergreen, flowers in spring and lovely red new growth. Or coronilla..I used to grow mine in a similar sized pot in a porch (open to the elements though) and facing East. It flowers from autumn until spring and the scent is lovely. Another nice one is lotus hirsutus or dorycnium which has downygrey-green foliage (evergreen) and white flowers with pink tinges. It only grows to 60cm. Or you could have a miniature standard rose? So many to choose from...I’ll stop now!

I had my Pieris in a pot for many years until I put it into the garden this year. If you do grow it in a pot do remember it needs ericaceous compost or it won't thrive. Lovely plants.
Good luck with whatever you choose.