UK to Japan then China then back to UK

I am flying to Tokyo in October then planning to visit Shanghai, Guangzhou (and preferably Shenzhen) before flying back to the U.K from Beijing. I am planning on being in the above cities no longer than 36 hours.
Obviously my question is do I need a VISA without visiting Shenzhen and if I do visit Shenzhen will this necessitate me obataining a full VISA?

Steve, Birmingham U.K.