The purpose of the 'five-a-day' plan is to get sufficient micronutrients into your diet e.g. vitamins and minerals.
Seeing as most nuts are nutrient rich, to that end, they could be counted as one of your 5 a day.
Almonds, for example, are a good source of several B vitamins as well as phosphorus and magnesium.
However, one of the other purposes of the 'five-a-day' plan is to get fibre into your diet. To that end - it would be best to include some more fibrous fruit and veg into your five a day.