If it were me, I'd book as 1 thru ticket as opposed to split tickets within the same PNR.

• You'll not be paying UK departure tax on the thru ticket
• Any irrops will cover the full trip rather than specifc segments selected.Example your YUL-LHR suffers significant delay/cancellation causing you to misconnect to your onward flight. You're fully covered on a thru ticket but this isn't the case if you're on split tickets even on the same PNR. You'd be on the hook for cancellation/change/no-show rules covering the LHR-India ticket if you're not there in time.

I assume there's a significant cost difference at play by split ticketing which is why you're asking the question. IME it's just not worth the hassle. Any cost saving will effectively be eaten up if any leg of your trip doesn't go ahead as planned.