For full shade there aren't many small shrubs that come to mind that are evergreen and colorful. You may need to rely on leaf colors for color. Green comes in many shades.
One of my favorites for that situation is Nandina. There are dwarf forms that grow to between 2' to 4' depending on variety and stay rounded. I love the lime green leaves that change colors in the fall and some in spring too. Not sure which ones would be available in the UK.
Nandina 'Nana Purpurea'
[url=https://www.hort.net/gallery/view/ber/nandowd/]Nandina[/url] [url=https://www.ces.ncsu.edu/depts/hort/consumer/factsheets/shrubs/nandina_domes-woodsdwarf.html]'Woods Dwarf'[/url](I grow these in my garden)
Nandina 'Gulf Stream' -
Nandina 'Moon Bay'
Nandina 'Fire Power' -
Nandina "Plum Passion'
Nandina 'Sienna Sunrise'
Hardiness zones in the UK are mostly 8 and 9 and you don't say where in the UK you are. It would be helpful to add that to your profile like I have.
Helleborus will grow in full shade, is evergreen and has a long season of bloom. Most of the flowers face down because it starts blooming in late winter when there is often snow. There are newer cultivars that have flowers that face up. Most are hardy to zone 7, so choose carefully if you are in zone 9. They usually start blooming in March and bloom until May or June in my zone 7 garden.
Liriope aka Lilyturf comes in green or variegated varieties. There's even one that is called 'Black'. They only need to be cut to the ground once in the spring before new growth begins. I grow these in shade.
I also grow dwarf mondo grass. It looks like grass and is lovely at the base of a rock wall or between stepping stones. It makes a nice green accent when used with other colorful leaved plants.
Cotoneaster is another woody plant that comes in all sizes - from groundcovers to very large shrubs with thorns. The groundcovers do have lovely berries.
Skimma japonica is evergreen with varying heights. You will need both male and female to get berries, but the berries add to the color. The groundcover height might be nice with the Nandina.
Camellia is another option. Most are slow growing shrubs, some are evergreen and there are varieties that bloom in spring and some that bloom in fall. You will want a dwarf variety that will stay small.
Hakonechloa is a lovely grass-like plant that will grow in shade.
There are many annual plants that only live one year and bloom in shade all spring, summer and early fall. They are frost tender and you would have to replant these every year. If you are interested I could give you some ideas. If you know your hardiness zone, that would be helpful too.