Technically I believe that you cannot tax it if you are not the owner. You used to be able to pop along to the PO with your green bit of the V5 and that was ok. Can’t see how you do that online as you will not be the registered keeper. Also its damn near impossible to insure a vehicle that you don’t own and you can’t tax it without either a cover note or it being on the insurance system.
I guess you get a cover note from your insurer before you leave home, maybe saying wither that you already own it or will do so from a certain time, take that and the little green bit to the PO and then drive home.