I must preface this with the reminder that this is how I personally would treat this new area and is subject to my own personal tastes.
We have a triangle roughly 18’x16′ facing mainly East with early morning sun, ph neutral to alkaline that has previously been laid to lawn. The requirement is mainly shrubs with some perennials, some evergreens and with mainly white flowers.
The ground will have been compacted under lawn for some years, the first priority must be to till and fertilise the soil. It is not a good idea to plant shrubs in compacted soil and merely dig out a hole for the roots, this effectively sends the roots in a circle and they will not develop to proper maturity, the soil should be hand dug or mechanically tilled with good additions of bone meal and organic fertiliser such as matured manure before planting should commence.
Consideration of plants.
The ph alone precludes the use of many stalwarts of the shrub garden. Most camellias, rhododendrons, azaleas, kalmias and magnolias will not tolerate the ph. Many shrubs, conifers and roses will not tolerate the lack of sunlight hours and the chemistry of the proximity of a yew hedge.
Consideration of design
Some time ago I’m sure pseu posted some pictures of her house, I remember considerable yew hedges of some height and a very traditional English looking middle class home, I do hope that these were your pictures pseu! Taking that into account I personally would not want to see any ‘tricky’ looking plantings, I would take care to avoid too many coloured and variegated foliages and go for a very traditional English looking border to compliment the structure already there. Yew is a dark and sombre foliage especially to the East with limited sunlight. I would use a limited palette of single green, grey/silver and bluish foliages with mainly white flowers with highlights in soft blue and pink to provide lift. I have considered gold and feel that it may not be so complementary in the setting, it would be perhaps too urban and may be construed as a bit ‘Golders Green’ for the environment!
Trees and shrubs
I think I would want to see a lift as a focal point, I suggest a tree, Betula jacquemontii, a Himalayan paper barked white birch to provide contrast in both texture and colour against the yew. Not to be centred but off to one side a little, underplanted with an offcentred oval of lilies of the valley.
Other shrubs, with the caveat that there is not too much room for them and that they must earn their keep by being long flowering -to provide winter interest, Viburnum bodnantense ‘Dawn’, this flowers white /pinkish all winter into early spring and has a charming scent, it is not over vigourous and has a very strong structure and interesting leaf shape. To provide spring interest in white, Spirea Bridal wreath and the summer, shrubby cinqufoil, potentilla, something -wood is white and flower until frost! I cannot see either hybrid tea roses or shrub roses being in scale with this planting, however I can recommend a mounding rose, Ballerina that flowers small pale pink flowers from June until frost, most reliable and only needs light clipping rather than traditional pruning. For the autumn perhaps hardy fuchsia, there is white single pear drop style blossom I think it is called Hawkshead, or maybe hibiscus, the satin range has a nice double white flowered one. For the winter perhaps Skimmia would take the ph and as long as you plant both male and female will have white flowers, red berries and is evergreen for interest.
Ground covers and perennials
I can see hellebores being most useful here, they are effectively evergreen, long blooming and have useful coloured foliage. Hellebore augustifolia has a silver grey dentated leaf which remains handsome all year and a natural green flower that last 4 months. Hellebore orientalis has a cultivar White Queen’ that is white flowered, fading to purplish with interesting seed heads.
Brunnera ‘Jack Frost’ is deciduous but has a green leaf with silver white tracery and sprays of pale blue flowers in spring, most useful for the shady part of the bed.
Stachys byzantium has silver furry leaves which effectively mats the ground with a pale pink flower .
Bergenia is evergreen and can be had in every colour from white to dark pink.
Hostas would make bold statements, I think that sieboldiana elegans, with its large bluish foliage would be effective.
Personally I would restrict the palette of plants and plant the perennials in groups for reasonably fast impact, the austerity of the background needs an elegance not a messy ‘bunged in’ look, structure and texture is all.
White crocus teamed with those pale blue stripy, spotted blue ones and snowdrops.
White daffodils, but beware they are beloved by slugs, prepare to defend them! Pheasant eye narcissii.
Ivory tulips, teamed with pale pink viridiana tulips and some black ‘Queen of the Night’, followed by ornithalagum or camassia alba for early summer.
Lilies, unfortunately all the asiatics and orientals will not get enough sun to do well here but there are lilium candidum that prefer neutral to alkaline soil and lilium regale that should do ok in the sunnier part of the border. Also toad lilies take a poorer light.
Cyclamen coum will take the part shade as will various of the anenomes.
When planted, I would make sure the bed is mulched with a fertile mulch, you will need a couple of yards to give a satisfactory depth of mulch to inhibit weeds and provide nutrition and humus, that soil will have had no added humus for years.