A camellia is evergreen and flowers in spring. Mine is only now coming into flower, but my neighbour's has been in flower for a month or so.
Pittosporum doesn't flower but has pretty leaves. P. Elizabeth has a lovely pink tinge.
Photinia Louise will give interest all year round - it is a red robin with a pink tinge.
Cornus siberica has striking red branches in winter.
Flowering cherries are £7 each in Tesco at the moment. They have shrub ones and tree ones. I bought a pink one and a white one, that I had been coveting on the Crocus website, same varieties, but just £7 each.
Eriseyum (excuse my spelling!) bowles mauve is a wallflower that flowers pretty much all year round.
Other easy shrubs which will flower and add interest are hydrangea(needs a lot of watering), buddleia, lilac, jasmine, rhododendron, fuschia, azalea, lavender - many more too.
When I was investing lots of time and money in my garden this time last year, I was told to buy evergreen shrubs that flower, and also look for things that had interesting foliage, so variegated and also not all the same shade of green. It served me well for the type of garden I was planting. This year I am adding interest with more perennials and annuals eg salvias (which I absolutely love) in various shades of pink and blue, scabiosa, lupins, foxgloves etc.