In a large saucepan, melt the butter over medium heat. Once melted, add the flour, whisking constantly until a paste forms and the mixture is smooth.
Gradually whisk in about ¾ of the milk. Continue whisking until the sauce thickens; you may not need all the milk, but keep a splash or two aside to adjust consistency later if needed.
Add in the grated cheddar, mozzarella, and Red Leicester cheese gradually, stirring after each addition until fully melted. Set aside a handful and a half of each cheese for topping.
Stir in the garlic powder, onion powder, black pepper, salt, paprika, and oregano. Taste and adjust seasoning if necessary. Turn off the heat once combined.