Yes. The most generous allowance on a booking will prevail for the entire booking.
There's also an interesting, if not beneficial "glitch" in the system:
If ANY leg of an itinerary on a booking is booked in what BA thinks is a premium class then the BA higher allowance will be given.
For example...many carriers use "Z" as a Y booking class, but BA uses it at "F". So, if there's Z class appearing on the booking...regardless of the carrier...BA will offer the "F" baggage allowance, even if booked in WT on BA.