I suspect just putting the reg on retention in order to keep it, then once you have the other vehicle at some stage just transfer the reg to it. Dependant on the dealership where you may be buying the replacement vehicle from, they may be able to have the change over carried out in one move for you if you are trading in and IF the reg is not deemed to be high value...
Been through the a similar process and the retention method was the route I took.. The process of initiation of the retention can be done on line very quickly, though the paperwork can take a while longer, but as long as the DVLA have the data the plates can be changed over and the vehicles used.. Assuming you have the original plates for the vehicles involved... I tend to keep the originals for just such circumstances..
hope this helps.
Good luck.
