On a practical level: People don't normally want to remember number plates because of your excellent driving.... it therefore seems rather foolish to have a memorable number plate - as it will only be remembered by people who have something to complain about (or report to the authorities)
Is fitting a private plate so much different to someone who names their house ' Bluebell End' or the likes, instead of using 131 Whatever Road like most other people do?
At one time, unlike many other European countries. the UK seemed preoccupied ( and to some extent still does ) with regard to new car ownership, when owners in certain circles were judged how successful they were by the age of the vehicle they drove, so it was a logical step for some owners to use a private plate to disguise the age of a vehicle. It's common practice today in private coach fleets etc to use them.
I would say that right now in the UK, given the state of the economy, a 'good or unique' private plate represents a much better longer term financial investment than many conventional investments as there will always be a strong market for distinctive number plates as fewer of these number plates become available and some people will always pay a premium to own them.