I had a similar situation 18 months ago. I suggest that you make a new application, for a “first passport” and send a covering letter explaining that your current passport is almost full, but has a valid China, or other, visa with a year left. Make sure that you give a UK contact number as they will then telephone you to say that it can’t be done. Explain over the phone why you need a second passport and the guy will give you his name and suggest that you write to him explaining again. He will then take your case to his boss and it should work. They may require a UK address to send it to by recorded delivery.
Hope this helps