Can Britain win against China in a battle? Yes.
Can Britain invade and take over China? No.
Can Britain invade China, take for example Hong Kong, and keep it? No.
Can Britain invade China, take for example the big island in south/Tibet, and keep it? Possibly.
Can China win against China in a battle? Yes.
Can China invade and take over Britain? No.
Can China invade Britain, take for example the Channel Islands, and keep them? No.
Can China invade China, take for example Pitcairn Islands, and keep them? Possibly.