I wouldn't be that worry about 50 min. as both flights are with Air China.
It should be possible.
My gf did it within 30 min. in august as her flight from Nanning had a huge delay in Beijing and she just had 50 min. left before her departure to Germany.
One ticket on CA.
She got pick up by CA staff at the aircraft and guided to the international gate through "not official ways" to the gate (incl. security check and immigration).
CA is aware about the tight connection and it would wonder me if they can't bring you on the London flight.
Your gf did not have intl to intl though