And what sort of economic pressure can Britain force upon China? It would take the entirety of South America, Africa, and South East Asia ending trade with China to actually bring it to its knees. Britain has no clout with these nations. At best it could hope to convince the rest of Europe to do this, but with the looming economic crisis it will be for naught. Even the United States may not be in favour of ending trade with China when the economic crisis hits.
Not unless the British blast PLA positions with tactical nukes.
But that monumentally bad idea would result in an nuclear exchange that leaves 35% of China's and 90% of the UK's population dead so it would probably be better just to crush China economically then use force of arms.