National treasures

There seems to be a lot of references to National Treasures recently. I don't have a problem with this concept, in fact, within reason I think it's quite jolly. Recent nominees for this epithet include David Attenborough, Bill Oddie, Michael Palin and Stephen Fry. This set me wondering what the criteria are for being a National Treasure. Based on the above selection, it seems to me they would include: being or having been a comic writer / performer /actor , and / or a wildlife presenter, and have been around for quite a long time.