You get the allowance from BA from DEL-LHR for the whole itinerary because they are the most significant carrier. A US Department of Transport rule further ensures that this will apply for the return trip also (even if the airlines weren't intending to do that for you).
If you book separate tickets you get the allowance on each ticket for each journey the ticket covers. Bear in mind that separate tickets get you no connection protection, maybe no interlining of bags through airports between tickets, and so on. It is up to you whether the cost saving is significant enough.
Caveat Emptor
Originally Posted by
UKtravelbear
how long is the stop over?
That is irrelevant as long as it is one reservation.