I agree with A7UFO on this one.
I have W4--- which I bought in 2002, I bought V7--- for my wife in 2003, then J6--- and L5--- which I am keeping on retention for our kids when they have their cars in 12 and 13 years time.
The most expensive was W4 at £399, and the others were £250. We will keep them for ever as they have our initials, and will become quite rare in years to come. Even though I pay £25 each a year to keep the kids numbers on retention, I think it is worth it as we won't be able to buy single digit plates by the time they have their cars. You struggle to find them now.
I think that as time goes on, prefix plates will become sort of dateless, as the gap widens between the age of car they go on and the year of the plate. Also because the new style private plates still have 7 digits, they dont really stand out as being special.