The film was never aired in China when it was first released. I'm not sure the government ever explicitly banned it I know they did make sure it didn't get shown.
The film carries a very clear anti-authoritarian message with people banding together to incite a popular uprising against a totalitarian government. The UK is just used as the backdrop. The film depicts a government cracking down on media freedom, imprisoning those that even mildly disagree with the regime using secret police forces, lies and deceit from the government. The parallels are obvious.
If you think that core message and the relation to their current situation is going to be missed by the average Chinese citizen then you have a very low opinion of them.
Already their social media systems and blogs are buzzing with quotes from that film that should fill that ruling party with dread. "People should not be afraid of their governments. Governments should be afraid of their people". Oh yea the government really wants that message out there in the minds of the average Chinese citizen.