A Star Alliance member Turkish airlines offer competitive solution from main airports in the UK to India. Cheap return flights trom London, Manchester. Birmingham or Edinburgh to Mumbai are available already for £279. This is inclusive of all taxes and fees and checked bag. If you are adventurer and appreciate multi-city solution you can book rarely seen multi-city deal that is actually cheaper than classic return flight. Multi-city flights from the United Kingdom to Mumbai with (at least) one day in Istanbul are available from just £251!
Turkish Airlines promo sale is available in limited quantity when traveling from the UK to India from now till 8th of December 2018 or 15th of January to 15th of March and 1st of May to 10th of June 2019. Max stay 4 months. Book this promotion by end of September or until sold out..You can compare all the cheapest options at website of Turkish Airlines. If you would need cheap accommodation in India you can use hotel booking agents of Booking.com or Hotels.com.
Booking sample of cheap flights from the UK to India (Mumbai) already for £251! In this case you would depart from London Gatwick to Mumbai with one day stop in Istanbul on 24th of February. Inbound flight from India back to London Heathrow on 11th of March 2019.