China's armed forces have numbers, but they are mainly conscripted, poorly trained and equipped compared to our own. China also has limited power projection of it's armed forces, so would require a massive investment in long range conventional weapons systems and a massive investment in its bluewater naval capability. In any war on Britain, without first securing land based supply lines through eastern and western Europe (which would require invasion or cooperation from Russia and northern Europe) then Chinese forces would be seriously out gunned and easily out manoeuvred by the smaller, better trained, better equipped and technically superior British Naval and Airborne forces.
The main problem when contemplating a Chinese invasion of Britain is that we have several treaties, NATO had been mentioned, we also have a mutual defence treaty with the United States which would require them to respond.
The most important thing to consider however is that in an official declaration of War and subsequent invasion China would be in effect declaring war and invading the EU, not only Britain. Economically and Militarily the EU are vastly superior to China, this would require China to commit forces to an invasion and huge resources to hold Britain if it were successful.
This would leave China itself vulnerable to attack from it's neighbours and more importantly it would leave it unable to respond to internal pressures from it's own people and would in all likelihood fall to a coup or descend into civil war very quickly.
In short, it is highly unlikely that China would either have the resources or political will to attack Britain, and even if it did, any invasion would be short lived.
I purposely haven't mentioned the use of nuclear weapons as that would change the demographic of an invasion into a World War of mutual destruction which is not what the OP implies.