Yes, quite legal. Found this:
MOT Test of Mirrors – Visibility (3)
View to the rear
This section is now covered under the new general section ‘Visibility’ (3), which includes Windscreens, and is repeated in the relevant section on this website here.
The Tester will check that all mandatory rear-view mirrors or indirect vision devices provide an adequate view to the rear from the driver’s seat.
Indirect vision cameras may replace mirrors on some vehicles. If such devices are fitted, the camera(s) and the screen will be inspected.
Rear view mirrors and indirect vision devices can be any of the following positions:
an exterior mirror or device that provides a view along the offside of the vehicle
an exterior mirror or device that provides a view along the nearside of the vehicle
an interior mirror or device which provides a view to the rear of the vehicle