Goodness me, what could we do? Start a Falklands style invasion! I assume that you don't want us to call on the USA in what might well turn into WW III?
Military force is not going to do it as we would be roundly beaten, and we no longer have the moral clout we did in 1945; that was deeply undermined in military style over Suez not long after. No chance of the UK bringing any pressure at all diplomatically or even economically on China without even the thought that any economic sanctions would be absolutely ruinous for us - far more than the Chinese. More like banging our head on a brick wall.
Best wishes from George