You're lucky - my mother has had a Britannia sandwich set since 1949 (a wedding present), the year it was introduced, and it was dragged out every Sunday for afternoon tea (and other times besides). Suppose I'm culpable, as I've since replaced the odd broken bit and even given her a crescent-shaped dish. The photo on the front cover, and page 21 of my book is one of her plates.
But although Britannia was the most popular range, and the most extensive set, there are still some very rare pieces I am still trying to track down: the octagonal* dessert and nut bowls, and the full 5-part hors d'oeuvres set for example.
*I've just realised that my book mentions these as being "hexagonal", so another one for the errata list.