Hi Gail,
I bet you're looking forward to your trip to China now as there is not long to go!
It is true that you cannot obtain Chinese Renminbi outside of China as it is illegal and very risky to do so because of serious counterfiting problems they have had in recent times! You will be able to change it securely in banks and hotels when you are there, and you can change it back again in China or Hong Kong if you find yourself with too much. The good news is that the exchange rate is the same wherever you go and there is no competition between places of exchange trying to offer better rates, so you know you won't be able to get a better, or worse deal wherever you go!
As for the Hong Kong add-on, you will be fine taking sterling travellers cheques or possibly a few Hong Kong dollars for when you get there. If you want to exchange money and get the best price for your pounds sterling, you will get a better exchange rate once in Hong Kong and there are also many places at which you can get your currency exchanged.
I hope this helps and if there are any other questions then please do not hesitate to email me.
Kind regards
Cheryl, your editor