Thank you for your comments and advice. Once again apologies for wrongly using the term "coupons". This will be one PNR, one Ticket (even with the difference in last 2 tickets sometimes). The only difference being whether I book YUL-India as one way or as multi-city YUL-LHR and LHR-India. As pointed out by most there would be absolutely no difference in respect of Immigration and baggage through check-in. Pricing is also the same. I find that the system does not add UK departure check either way. Only difference which may arise is in Schedule Change or IRROPS situation as pointed out by 1Aturnleft.

A. In case of IRROPS/Schedule change prior to departure from YUL, would BA insist on my going to India on some other routing if YUL-India is booked as one journey? Will I not be able to insist that I touch LHR and have my desired layover?

B. Conversely, in such situation if I wish to bypass LHR but I am booked ay YUL-LHR as one journey and LHR-India as another (same PNR, same ticket though) would they force me to touch LHR even though I may not wish to at that time and insist that I be carried through to India on any airline, any routing?

Many have said it is all the same, but when I go to the confirmation screen it shows the flights in two different ways. I am trying to attach screenshots from booking screen. One shows Journey as YUL-HYD in the header, the other one shows journey as "YUL-LHR" and "LHR-HYD".

Does it matter or are my rights as regards A & B same?

Thanks once again!