ΔE 7.3, close: similar, with a visible difference side by side. Middle Stone is a little lighter, more saturated, cooler than British Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Middle Stone is a little lighter, more saturated, cooler than British Brown.
The same difference applies the other way round: British Brown is a little darker, more muted, warmer.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See British Brown on the colour wheel See Middle Stone on the colour wheel
All matches: British Brown equivalents · Middle Stone equivalents