Without the hightop roof fitted the DVLA will not entertain the idea of changing your V5 to show the vehicle as a campervan.
However, all is not quite lost. While the V5 gives the registration type of the vehicle, it does not have a direct effect on the speed limits. Those are taken from the C&U regulations.
Take a look at the article here from Jerba Campervans, it explains it all quite well.