I’m not sure if the UK is the same as here, but in Australia the reason you have both for major roads is that generally you have regular names, and then the let

I’m not sure if the UK is the same as here, but in Australia the reason you have both for major roads is that generally you have regular names, and then the letter/number is the route designator. A route will generally traverse multiple streets/roads/motorways. So in the state I live in if you follow the M1 you will go from the Pacific Motorway at the southern border, onto the Gateway Motorway and then on to the Bruce Highway. So they have different roles.