I think you should keep your idea of some evergreen shrubs for all year interest
Choisya woudl be fine and fit nicely in a space 1.5m . The yellow ones are not very hardy if you are in a cold part of the country, great if you are sheltered. All the green ones are lovely
viburnum tinus Eve Price can easily to pruned to keep in that space , they have year round interest
There are lots of hebes, with different leaf colours and sizes , they will stay reasonably compact . I use the small leaf ones and trim into big domes .
Skimmia are good for winter interest
Nandina is evergreen in warmer areas
Spotted laurel will grow to big for 1.5 m and the leaves are so big it's hard to prune it and keep neat
Photinia can be easily pruned also it's not a very attractive shape . But lots of people love it for the colour
There's also dwarf conifers if you like them
There are many evergreen grasses . And some smaller phormuim would do too . They will still look fine it they grow slightly above the fence hight
All of these are good on clay . You can add deciduous shrubs, perennials and of course bulbs