Even at its reduced growth rate, China's GDP is growing so fast that it will add the equivalent of the UK's entire GDP in just over 3 years. That's why we need the EU. We are minnows swimming with sharks when it comes to post-Brexit trade negotiations! 
Even after Brexit, the EU will be at the top table of trade negotiations. We'll be sat at the little child's picnic table in a draft near the door.
Worth also noting that the legal wording of the recent trade deals signed by the EU effectively prohibits better trade deals being offered to any other country. So we literally can't better the trade terms the EU's getting, no matter how long or hard we try!
Some good general background to this here...
prospectmagazine.co.uk/politics/the-m…
prospectmagazine.co.uk/politics/the-m…
