Hopefully helpful
So here are my 2 cents
You will probably fly out of LHR on the AI dreamliners. Service is decent on these as is the crowd as most are connecting from Canada. Flight should be relatively empty post 5 January. The 787 crew are the non unionized Delhi based ones & relatively younger than the ageing aunts who fly the Mumbai 777 routes.
You will arrive into Delhi T3 & based on your nationality are or not eligible for an E visa that would need to be applied for earlier on line. If you have one of these you can step out of the airport & visit the city. You will need your boarding pass & ticket to re enter the airport. Since your bags will be checked to CMB itself you will not need to collect your bags unless you specifically ask for it. If you have a UK passport the visa will cost a bit (reciprocal) & it is not worth it.
The flight to CMB will be on an aging 320 from the same terminal. As some posters have suggested if you want to transit only your bags will be checked thru & you ate best off taking a nap at the transit hotel. Currently T3 is going thru retail renovation with only a spattering of food places open & not much to do for 13 hours. Fog is playing a spoilsport & if you Are lucky your flight from LHR could get diverted to Mumbai & you arrive 10 hours late in Delhi just in time to catch your flight to CMB.
Bon Voyage!