As silver birch look so good in winter when the sun is on their stems, I'd emphasise the interest with good evergreens such as holly which can be plain green or variegated, viburnum Eve Price which has flowers in winter, skimmia ditto. For the holly and skimmia to bear fruit you need one of each sex in the neighbourhood or garden.
I'd also plant some shrubs with coloured stems that glow in winter such as cornus alba sibirica - bright red - and cornus Midwinter Fire - flame like - or some of the colourful forms of salix available. These will need to be completely or partially pollarded (cut back) each spring to maintain fresh coloured new stems. There are also several forms of small conifer in shades of green, blue and bronze which will add further interest and make a good backdrop for other plants. You can trim them to keep them to size.
Once the shrubs are in I'd put in a mix of perennials with contrasting heights, leaf forms and sizes. If you do some research, you can have something in flower every month but most perennials will die back underground for the winter and thus leave the evergreens and bare stems on display.
Plants such as hellebores will add winter interest with foliage and flowers in late winter and early spring and there's a whole host of bulbs for spring colour too starting with winter aconites and snowdrops through crocuses and narcissus which can all be naturalised.
Before planting anything, prepare the planting holes or new beds well with plenty of added garden compost or well rotted manure and sprinkle bonemeal in for roots development. The best time to plant shrubs is autumn when the soil is warm and the roots can grow away quite quickly. You can plant pot grwon shrubs and perennials now but you nee dt make sure their root ball is thoroughly soaked first and taht you keep them watered throughout the summer so they don't get stressed.