I have an Irish passport, but born, live and work in England. Can I get a visa in London?

Hi, I'm planning a trip from London to Shanghai on January 26th to February 10th then I plan to travel to Cambodia, Thailand and Vietnam until February 20th. I then return to China for a day then fly back to the UK on the February 21st. Will i need to get a double entry or do I need a transit/ not required at all for staying in Shanghai for a day?