I haven’t tried Sainsbury’s but have tried Marks and Spencer’s frozen Indian dishes. It’s passsable if you’re desperate and missing it but the quality and taste isn’t great imo. I think some other food like Italian lends itself better to being reheated.

With respect to restaurants, I’m in central London, so not a suburb with many Indians, and there’s some decent places like Dishoom, Roti Chai etc but I wouldn’t say they’re cheap. Im thinking outer London in the west and then places like Birmingham probably have cheaper places but I never go out that far.