Single or Double entry visas

Hi, I am a British and I am planning to visit Shanghai for 2 weeks with a return ticket back to London. During my stay in Shanghai I am also planning to visit Seoul, South Korea with another airline's return ticket. I will be travelling back to London on the same day I arrive back to Shanghai from Seoul. Do I need Single Entry or double entry for my whole journey? Example below;
1) Single Entry for entry to Shanghai from London and use 72 hours free transit when I am back to Shanghai from Seoul as I will be travelling back to London on the same day
OR
2) Double entry for both entry to Shanghai from London and from Seoul.

And what type do I need to apply before travelling to Shanghai?

Many thanks in advance,