Well done, Swanhunter that's a great guide!

Here are some of my additions:

Arrival - although the BA flight does disembark at a jet bridge, it seems to get further away from Immigration each time, so you can have a good 10 minutes walk. Which is quite handy as you can give your legs a stretch after the 8 hour flight.
I haven't seen the landing cards being given out at the top of the jet bridge - I usually arrive on the BA199 and there are usually just a handful of porters waiting with wheelchairs. However, all is not lost, there are usually stacks of cards on the writing desks when you get down to the Immigration hall.
Luggage retrieval time is patchy, mine has taken less than 10 minutes on one occasion and more than an hour on another.
That's an important point about keeping the little tear-off strip at the bottom of the landing card. Once you pass through the baggage screening, you will see a little knot of people frantically searching through pockets and bags to find it. I doubt if you can go much further without it.
If you do need a taxi, go to the prepaid taxi desk and get a "Cool Cab". These have airconditioning and seat belts (both usually functioning). They seem to have chased most of the taxi touts and beggars away from the car park these days, but as Swanhunter says, be aware, especially in the early hours.

Departing - (Following applies to WT+ and WT pax only) For the overnight flight, the check in opens about 22:30. OLCI is strongly recommended - not many people are aware of it in India and you can get a good pick of the seats. I generally aim to be at the airport between 23:00 & 23:30. This avoids the first rush at check in and also allows some of the earlier European flights to clear out. If you don't have lounge access, you can bag one of the couches to stretch out on. The quietest place is past the duty free shop on the right, just by the door for Transit passengers. Talking of duty free - don't bother! There is rarely much choice and what there is is overpriced anyway. Facilities are not great, but they seem to be working on it as there always seems to be construction going on.
Keep your eyes and ears open for when the BA flight is called for "Security Check" around 00:30 and get there before the rush. The security points are at either end of the lower level and in the middle. the middle is the worst one as there are invariably 2 other flights leaving from these gates and the queue snakes back to the escalators. Patience is a virtue in this case (and if you have spent much time in India, you will have this virtue well practiced). Even if you have been shuffling in the queue for 1/2 hour, let the aircraft crew in before you - they use the same x-ray machines - especially the BA crew. Ladies and gents have to go through different portals either side of the machine. Put your mobile, keys, loose change etc. in your bag for x-ray.
Once you are at the gate, hopefully you will be early enough to get a seat near the door because as soon as anyone with a BA uniform shows up, people start to gravitate around the door. As soon as the priority boarding is announced, herd mentality takes over and the door gets bunged up, with a small path reluctantly cleared for the premium pax, along with a few chancers trying to get on before everyone else. Boarding by seat row number is usually attempted, but they generally give up in the face of overwhelming odds.