Preheat the oven to 200°C, fan 180°C, gas 6. Add the macaroni to a pan of boiling salted water and cook for 10-12 minutes. Add the peas for the final 2 minutes of cooking time.
Meanwhile, melt the butter in a pan and stir in the flour with a wooden spoon. Slowly add the milk, stirring continuously. Bring to the boil and cook for 2 minutes, then stir in the ham and cheddar.
Drain the macaroni and peas, then stir through the cheese sauce. Pour into a small baking dish and top with the breadcrumbs. Bake in the hot oven for 10-15 minutes, until golden and bubbling.