For an excellent crop (brassicas are really well suited to the British climate!) you just need to put a couple of precautions in place to keep pests at bay.
1. When planting, pop a brassica collar around the base of each plant. This can be a simple 10cm carboard disc with a slit cut into the centre. It will help to prevent cabbage root fly.
2. Cover rows of plants with horticultural fleece or insect proof netting to keep cabbage white butterflies and pigeons away.
3. Plant nasturtiums in a separate bed nearby as a sacrificial crop. Cabbage white butterflies will be attracted to the nasturtiums and hopefully will stay away from your brassicas.