Do shops and restaurants in Guangzhou accept credit cards (Visa and Mastercard) issued in the UK?

larger shoppping centres and the westfield type ones you'll find probably all shops will, small shops and street sellers not. most banks these days will allow limited daily withdrawals direct from your overseas accounts via visa mastercard. plus symbols and citrus and a few others. like western banks look for the symbols at auto tellers and inside banks. take your passport.
Some shops that may have been stuck with not paying or denied c.cards have own shop limits on purchases if you find that occurring negotiate with manager to split the invoice into payable sizes, and or get an approval from bank. Not all recognise if cards have limits so they impose their own.