Yes indeed, I was going to mention goat as it's rather similar to mutton, being fatty and strongly flavoured and relatively tough - needs plenty of cooking - but unlikely to be encountered much over here. Can goat meat even be sold legally in the UK?* Our goat farms are all dairy aren't they? I mean to say I've had goat in Caribbean restaurants but never seen it on sale, even in a Cash 'n' Carry or ethnic market (maybe that restaurant 'curry goat' isn't actually goat but sheep?)
As you'll know, most 'Indian' restaurants are run by Pakistanis / Bangladeshis*, who tend to be Muslim and yet not being subject to Hindu dietary restrictions will cook you a beef steak but not use cow in a curry - or at least they'll balk at identifying it as such, so maybe it is considered a dirty or substandard product as you suggest, aside from the perceived sacredness of the living animal. I'm a little troubled by the halal business although many people aren't.
* An Indian friend (non-observant Hindu) of mine sometimes gets very aerated about this as he can't take his drink and is a
massive racist

I've seen some photographs of British 'Indian' restaurants from the 1950s and the term that was often used was 'Indo-Pak' - to make it clear that they were providing food from the Indian Subcontinent and not pretending to be authentically Indian as such. Indeed many restaurants advertise 'tandoori cuisine' for the same reason....and yet my nationalistic pal is correct: there are 'Indian' restaurants run by people who've never even been to the country, though this seems fine to me as any potential customer knows it's a kind of vague euphemism.
Mmm, mutton.
*Checked: yes goat flesh easily available if you go looking for it.