Once all manufacturing is controlled by China, they could refuse to make cars with steering on the right or use Standard measurements, increasing pressure and eventually forcing the UK and US to switch.
It also sounds completely plausible that a binary choice like left hand or right hand driving can be switched in a way that drives a pressure campaign that nations will give into China on because it’s an easier short term choice and wouldn’t gain an immediate reaction to due to the enormous cost that would be rapidly felt.
Mail carriers in the USA also have their wheel on the other side so they can reach people's mailboxes easily.
PS Your 240V electric grid works that way because when the US switched to 120V (in like 1900 or so), we sold all our old gear to you and you never switched. So now all your electric things cost more as a result and your electrical outlets will kill you. So your decision making on such things is suspect.