If LHR receives even one inch of snow they traffic virtually stops - majority of BA short haul flights get canceled (as BA is protecting their long-haul flights). LCY is even worse. I would not risk flying on 2 separate tickets in December. Please remember: BA will through the baggage through (if you ask nicely), but traveling on 2 separate tickets DOES NOT GIVE YOU ANY RIGHTS in the case of IRROP.