Can't see it being a regular AN-12 if it's departing from UK shores.
AN-12s in the UK tend to do ad-hoc flying - rather than regular schedules. Things like AOG, horses, humanitarian, oil etc etc.
Plus, there really aren't many An-12s serving Western Europe anymore. Maybe 4 operators with 6-10 machines? Bright Aviation are long gone... but it could be one of their old machines.
I'm thinking it must be something else though. Electra, Herc or a military machine.