If I understand you properly, no never tell BA about any additional tickets.
You want to go DUB-LHR-LAS but return LAS-LGW-DUB?
it might work IF you can get LAS-LHR then an airport change to LGW for the LGW-DUB flight as they would have to give you your luggage to switch airports, just make sure they don't have your FF number on the LGW-MAN flight.

I've done that myself, albeit a number of years ago so things may have changed regarding the luggage.