I was in London at the end of last year and visited the Imperial War Museum which is just great. But I also heard about another smaller museum next to the Royal Chelsea Hospital which is financed by the Ministry of Defence. And it was a pleasant surprise.
Although it doesn't have an exhibit hall full of tanks, planes and V2 rockets like the aforementioned IWM, the National Army museum focuses solely on the British Army from 1066 to the present day. There is a stunning collection of uniforms, militaria, diaromas and the occasional vehicle.
I've written a review on my website below and included some photographs and there's also a link to the official NAM site if you need further information.
But if you are in London I would definitely recommend it. The National Army Museum is not included in the major tourism guides but it's a hidden gem and you won't regret spending the best part of a whole day there.
And it's free to get in too.
The Waterloo diaroma is out of this world BTW. It's almost as big as my entire living room.