If you have GBP, and want to exchange into RMB in UK or China, You can just compare the RMB exchange rate in UK, Here is the link
Chinese Banks in London
GBP/RMB Exchange Rate in China Local Banks

If you have USD, it is better to exchange RMB in China local banks(such as BOC, ICBC), because China has the hugest exchange volume in the world for RMB service. In China, you can pay a limited spread for buying or selling RMB when you exchange currencies in banks. Usually, you don’t need to pay any handle fee to bank even you only exchange for 100 RMB(CNY).
If you need small amount to pay taxi or metro fare when you arrive in China's airport, you can withdraw RMB with debit or credit card, ATM is everywhere in China. DO NOT exchange your money on airport money changers, absolutely bad rate and need a additional handle fee(60-80RMB). Also, to exchange cash in China, you need a passport.
USD/RMB Exchange Rates with China Local Banks
7 Ways to Exchange Currencies in China