I'm no expert, but if I saw that for sale for £200, I'd buy it.
Raise you £50.
Very rare pistol.
Very rare in US where they were made, and in that condition!
US "Blue Book" value in 2014 for one in 95% condition (no mention of box) was $600 (= about £427). 60% condition $300 (but no mention of box).
So I'd say at least £250, maybe more - especially if you put it in an auction, and got two keen buyers trying to get it.