Hi Kirsty,
I did this trip in 2014 and obviously took Yuan currency with me. Towards the end of the holiday I needed to change some GBP notes at the hotel reception, which was fine but they MUST be in pristine condition. So, no tears or writing are accepted but as long as your notes are perfect you should not have a problem exchanging Sterling in China.
Having said that, I did have a problem with £5 notes simply because they'd never seen one before, but once I'd pointed out the Queen's portrait they were happy to accept them. Anyway, if you take £10 and £20 notes you will be OK as long as they are in good condition.
I hope this helps and if you have any other questions please feel free to ask.
Cheers
Mike