If you want to design your own recipe I'd switch the dark extract to pale. That will give you full control over what goes into your beer (who know how much of what Muntons puts into their extract to get it dark?). I think you'd actually end up with a beer that is too dark to be a brown ale anyway with that recipe.
Your changes sounds fine (although I don't think you need the caramel for coloring), although if you switch the dark LME to pale I'd double your chocolate malt to compensate.
My favorite English yeast is 1968, brings out the malt, not too fruity, ferments fast, and flocculates quickly.
For priming I'd probably go lower than 3/4 of a cup, use a priming calculator that takes into account the actual final volume of beer and the desired amount of carbonation (English ales are usually less carbonated, but its up to you).