If you have never been to India, it is a cliché but it is different from anywhere else. The crowds, the colour, the temperatures, the air, the noise, will all hit you - if you arrive at the wrong moment so will the rain or the humidity or the heat.. Airports have improved in the last few years (particularly the big international arrivals halls in places like Bangalore and New Delhi) - but be prepared for the numbers, the walls of people greeting the arrivals with pickups from hotels, companies and families, the mad cap traffic and the often alarming roads..
Lots of flights into India from the UK arrive in the middle of the night ... take this into account, invest in a relaxing first day when you arrive, get to know the place slightly.
There are many direct flights from London and prices seem reasonably competitive - but it is worth shopping around.