In the absence of Little Red, Star Alliance are happy to book connections to LHR with BA.
Not just United, many Air Canada fares via BA shuttle to LHR undercut their own Direct Rouge service, presumably to fill up all those 777 & 767s they operate from LHR.
Out of the carriers serving the NYC market, CO, and now UA have tended to be the most leisure orientated for MAN. They acknowledged it as much themselves.
They'll quite happily route that kind of non-FF customer through FRA or MUC.
I don't believe they see any immediate requirement to upguage or get people down to LHR via another alliance as much as, say AC, just to get their FFs on UA metal if the 80 and 101 happen to be full, as they regard current 2 x 757 capacity as matching demand.
I wonder if they'll suffer the most from the arrival of Thomas Cook though.