From NHS guidelines;
Do potatoes count towards 5 a Day?
Potatoes are a vegetable, but they don't count towards your 5 a Day.
Potatoes are classified nutritionally as a starchy food. That’s because the main nutrient in potatoes is starch (carbohydrate). Also, when eaten as part of a meal potatoes are generally used in place of other sources of starch, such as bread, pasta or rice.
Other vegetables that don’t count towards your 5 a Day are yams, cassava and plaintain: they are also usually eaten as starchy foods.
However, other root vegetables such as sweet potatoes, parsnips, swedes and turnips do count toward your 5 a Day, because they are usually eaten in addition to the starchy food part of the meal.