I suspect Tata will keep the plants open until it's politically acceptable to close them (2 years), or British costs have fallen below those of manufacture in India plus transport.
I wouldn't make lazy assumptions about Indian technical ability though- they are improving rapidly. And if they haven't got the capital to upgrade and innovate, well neither have we. That's one reason why all our car industry is owned by foreigners. the other reason was in another thread, and I don't want to get that going again.