A complete guess on my part, but given the plane will need some technical inspections before being allowed to depart back to London, G-VIIM is due to service today's BA203/202 Boston service. It shows leaving on-time for Boston, but hasn't been booked in for the return BOS-LHR sector (but BA202 is still showing as operating). I wonder are BA Ops wondering if they can position G-VIIM empty from BOS to YYR and return to LHR, landing tomorrow morning. In the meantime, BA202's passengers will need to be shared out amongst the few seats remaining on tonight's other BOS-LHR services or move them onto tomorrow's flights (which have spaces). The other option is G-VIIS (which is due to operate the late BA239 LHR-BOS service) is used as a rescue plane (and tomorrow's BOS-LHR day flight is cancelled) or it operates a delayed BA202 service while G-VIIM performs the rescue mission!

The joys of airlines Ops!

Pilot37