It is much easier to take your plate off your current car before hand so that the dealer can register the new car using the retention certificate as soon as he is able. It will cost you an extra £25, but well worth it so that you can have it on the new car straight away, and not have to wait any longer. You've all waited long enough for your cars as it is.
My Dad has just changed his car and left it for the dealers to sort out. It took over 2 weeks to get the plate put onto the new car.
I have a new plate I bought a year ago and will just need to give him the entitlement but I assume the retention document will do the same job..
You get a years retention when you buy a new plate, so make sure it's still in date. You may need to pay another £25 if the first year has run out before your new car is ready to receive it.