Because BA are based at LHR. If it wasn’t going to work for them, they wouldn’t have bid for the work.
Geographically, flying North is the most direct routing. A direct flight isn’t possible due to the Himalayas, and ULH cargo isn’t feasible as it would be load constrained. Flying via ANC would add another 500-600NM of ground distance, and whilst it would probably result in a slightly more efficient routing once winds are accounted for, it’s so marginal that it would be outweighed by the benefits of flying via LHR - crewing flexibility, engineering on site, no requirement to position crew to/from ANC, route unfamiliarity (oxygen escape routes over Russia, Alaska etc), overflight permits etc.