The baguette earns a separate entry in a British catalogue because of one structural fact: its crust. A baguette is built around a hard, brittle, shattering shell over an open, airy crumb, and that is the opposite of the soft floured roll that defines most British sandwiches. In a British café the baguette is the format you reach for when you want the bread to be loud, to crack and resist and shed crumb, instead of quietly yielding to the filling the way a bap does. The crust is not packaging. It is the texture the sandwich is sold on.
The craft is keeping that crust intact long enough to reach the customer, because a baguette's defining feature is also its weakness. It is split along one side rather than cut clean through, so the crust acts as a hinge and a wall that holds the filling in a trough. Butter or a soft cheese is spread to the cut faces to waterproof the open crumb against a wet filling, because that airy interior soaks fast and a baguette gone soft has lost the only thing that made it a baguette. The fillings are chosen to suit a bread that fights back: firm cheese and ham, brie and grape, tuna mayonnaise, chicken and bacon, things robust enough not to be lost against an assertive crust and dry enough not to flood it. It staling within hours is the reason a café fills baguettes to order rather than in advance.
The variations are mostly a question of what the trough carries and how long before it is eaten. The cheese and ham baguette is the café baseline; brie and bacon, tuna and cucumber, and chicken Caesar are the standard fillings the format has absorbed. The part-baked supermarket baguette is the same idea engineered to be finished at home so the crust is crisp at the point of eating rather than hours stale. Each deserves its own article rather than being crowded in here.