The British Warm is indeed a very useful overcoat! I can definitely agree that it is an ideal overcoat for colder temperatures. With regard to its tradition, it should always be made of a heavy (at least 34 oz per yard) Melton cloth and never of cashmere.
In case you want a new one you will most likely have to consult a tailor or provider of military apparel. However, every once in a while you will find a fantastic, original British Warm in London's top notch second hand stores.