It’s super easy nowadays, I put my plate on retention off the Range Rover on the Wednesday, paid the £80 fee and by Saturday the V5 and retention certificate had arrived.
new car delivered Monday, V5 arrived on Thursday and by 7pm the plate was in the new car and no admin fee from my insurers.
when you purchase your plate the first time the first £80 transfer fee is usually included
(keep your old plates as when you retain the plate your car usually goes back to its original reg no)
as someone has already said it can all be done online now but strangely only between 8am and 7pm or something obscure like that