Would you delete the sub-categories
31190 Stamps Great Britain : Commemorative : 1960s
31191 Stamps Great Britain : Commemorative : 1970s
31192 Stamps Great Britain : Commemorative : 1980s
31193 Stamps Great Britain : Commemorative : 1990s
31194 Stamps Great Britain : Commemorative : 2000s
31189 Stamps Great Britain : Commemorative : Pre-1960s
This is the kind of obsessive division that appeals to collectors. Buyer behaviour is quite different, they prefer to browse commemoratives across the decades because the chances are, they do not know what decade they are looking for, just something they do not have.
If you keep the sub-categories the number of stamps is likely to decrease because buyers will not be bothered to jump from one decade to another.
If you are not minded to delete the sub-categories, may I suggest you reinstate a general Stamps Great Britain : Commemorative : for buyers who want to buy and sellers who want to sell.