A recent episode of TV’s ‘Rip off Britain‘ highlighted the substitution of other meats for lamb in a comparatively high proportion of UK Curry houses. In fairness, it wasn’t horse, rat or rhinoceros but either beef or mutton. Now if I was a Hindu then beef would be a religious insult so I can understand the concern in that direction even though the blind testers seemed to thoroughly enjoy the substitute.
As regards ‘mutton’, when cooked over a decent period, it is as tender, if not more so than lamb. So in a carnivorous sense a bit of a storm in a baltibowl, in reality. Incidentally, most Balti Houses will advertise Balti Meat which will be mutton but usually none the worse for that.