A couple UK bmrs positioned in Calcutta could definitely help pick off those lone Japanese destroyers for sure. UK sub(s) would also help protect your expensive bmrs when attacking w/bmrs, or could be used to bait Japanese dd’s to isolate them, if you can build subs in India (or send them from Africa). You need to have India stay in the game though to be used as a launch pad, so they will need to be using most income for inf and def units in the early rounds. I agree w/Tirano that UK bmrs could be sent over from Europe, but I would prob send the London bmr over (maybe after a Taranto run) instead of building one in S Africa on the first turn. I’m just a firm believer in max def for London on the first turn (or save some income), and don’t generally build luxury units, or ICs in Africa unless the Germans drop all ground G1. Anz bmrs in both Queensland and India have good range for this mission (bmrs can fly back-n-forth from those ABs).
Another thing is that this form of sub ware fare takes a few turns to develop so you really don’t need to have UK build bmrs right away. You should max def both London and Calcutta, while the USA gets fleet and subs into position, then build the bmrs (which have great range) just before the US is ready to go IMO to coordinate efforts (will also help to hide your intensions). I like to have some mobile units built for the India, and this would fit into this type of swamp Japan strat because once the Japanese have to back off Calcutta a couple mech with those bmers can be pretty effective getting into China too.