As sandy soil on a slope will lose water quickly I would maybe go for some varieties that hold up well in that soil type.
Some grasses might help to anchor the sand and help reduce water loss.
For groundcover maybe Oregon Stonecrop or Moss Phlox (although the shade may limit the Phlox).
Flowering Quince loves free draining soil and possible bulbs that may grow okay in sandy soils could include foxglove and bluebells (although I think you would have to dig in some good ).
Mahonia 'Winter Sun' is a good shrub for the shade and likes free draining soil (again would add some humus/good compost). It can grow throughtout britain although can get a little big- although bounces back well from a good pruning now and then!
Hope you find plants which suit your needs!