I've flown this route extensively in all 4 classes and the night departure from LHR (BA199) and midday departure from BOM (BA198) are my preferred options because you avoid the rush at the Mumbai end.

I haven't flown the 789, but I personally find the difference between F and CW, ex BOM, (on the 777/747) to be fairly mild.

The food is not much different (the worst meal I've had in ~35 return trips on this sector was in F, ex BOM) , the alcohol is slightly better (but no pre departure alcohol).

The seat is better (the 789 may be better still) and the service is slightly more personalized.

You will not be able to use the CCR on your outbound sector if you upgrade your inbound sector.

You haven't mentioned you FF status, but BAEC Golds in any class get access to the F section of the GVK lounge in BOM which gives the CCR a run for its money.

The AUP price CW to F, for this sector, hovers around the GBP 400 mark.