one other reason
BA uses (used??) a system called FICO (cant remember what it stands for,) from which one could call up a particular aircraft by using the "last three" letters of the a/c registration. Using this system one could find out where a particular a/c was, what its rotation was for the given period, maintenance status and so on.
Therefore in the current fleet each must have a unique "last three". When BA reserve a batch of new registrations from the CAA they would,I suppose take this into account.