Maybe they should have just underwritten UK investment in the first place
With what? The same unfunded, cashless promises that underwrite all the other commitments that the UK bunglement has made over the years?
Because our idiot, idiot politicians signed free trade agreements without caveating them with a requirement for a balance of trade, the West has got progressively poorer in cash terms (fundamentally wrapped up as accumulated private sector debt, made worse by government deficit spending). Meanwhile, having exported but not imported China has foreign exchange reserves approaching four trillion dollars. It could spend that imports stuff from the West, but prefers to invest it in assets - so rather than buy Jaguars off the UK, they'd rather buy what will be the most expensive power station in the history of the world, and then collect rent off us forever.