Richard,
You can't just transfer any number about as the rules are pretty tight.
If you had a car with a reg you like but had been off the road for years then that car would need to be returned to a condition where it could be taxed, i.e mot'd and insured. Obviously the cost could run into thousands which is why is why people ring older cars.
I have heard of a particular chap (who I don't know) he has p4's and strips p4 for parts etc, when he gets one with a nice transferable plate he unscrews the vin plate switches reg plates gets it tested and hey presto.
I copied this wee bit from the DVLA site.
"vehicles must be currently licensed, subject to annual testing and available for inspection. However, applications involving unlicensed donor vehicles will still be considered if:
* the licence expired no more than six months prior to the date of the application; or if
* the donor vehicle is not old enough to be tested, i.e. less than 3 years old for cars, motorcycles, etc or less than 1 year old in the case of HGVs.
In both the above circumstances, all other requirements of the number retention facility must be met in order for applications to be considered.
PLEASE NOTE: Receiving vehicles must be currently licensed in order to participate in the number retention arrangements."