What is the route?
And if you are originating in LON why are you considering a three hour + narrowbody flight?!
LHR-MCT on WY is much more convenient than LHR-HEL-MCT.
If the choice were LHR-AUH-MCT vs. LHR-MCT I would go with BA for the Avios. But LHR-MCT would win every time over LHR-HEL-MCT.