Its not the N1 / M1 that dictates speed limits its the body type.
Motor Caravan = car speed limits which is why the aftermarket converters were up in arms last year when the DVLA registration rules changed.
Previously, builders van + pop top+ windows, cooker , bed & tables could have its body type reclassified to Motor Caravan & the speed limits change. Now (according to the rules) it needs a high top not just a pop top.
See extract from below 30th April Revision of the rules:
Motor caravan external permanent features
This list describes the external features which are commonly seen in motor caravans, and it is intended to provide guidance on what DVLA expects to see when considering your application:
- 2 or more windows on at least one side of the main body (this does not include windows on the driver or passenger doors) to provide a reasonable amount of daylight into the living accommodation
- a separate door which provides access to the living accommodation of the vehicle (this excludes the driver and passenger doors); a window on this door counts as a separate window on the main body
- motor caravan-style graphics on both sides of the vehicle
- an awning bar attached to either side of the vehicle
- a high-top roof (this does not include a pop-top elevating roof)
An ocean / coast does not comply with the above so shouldn't be registered as a motor Caravan.