As said before, with the twice yearly plate changes, it doesn't make that much difference these days.
Most people buying a 3-4 year old used car will buy on spec, mileage and condition over what number plate it had on it. If you px it, the dealer will tell you the older reg is worth less, but you'll have no trouble selling a nice car privately at that age, so should get way more than a px value. Plus with cash, you'll be in a better negotiating position for your next car.