It has been almost 2 years since my last trip but here are some things to be aware of:
1) Visit your doctor or local clinic for shots and meds before you leave if you have not done so already.
2) You will have to reboard the plane in London even if you have a direct flight.
3) Be prepared for Delhi Belly when you arrive. Local doctor and prescriptions should be very inexpensive.
4) IME the personal space awareness if very different then in the US.
5) I recommend a visit to a small island which is a short boat ride from Mumbai. Don't remember the name but the boat leaves from the "Gateway to India" There was also an area in one of the parks where the monks had their living quarters carved out of solid granite.