My car has P plate holders, though the back room broke off and I now stick the plate in the back window. In the both the 4WDs I put it behind the number plate and under the canopy door on the Navara. With the Hilux I have to tie the bugger on so I usually don't bother unless I am going along way or somewhere I think there might actually be a cop who cares.
I used to have magnetic ones, but I can't find them. Anyway, I could only use them on the Navara, I had one on the bonnet that blew off in a strong wind that I was driving into, but the one on the back never fell off.
It'll be good to be rid of P plates because they're a nuisance and you get the occasional idiot at uni who thinks it would be funny to steal them. I keep a stack of spare ones in the car so it's not a big deal.