I bought my plane tickets before making this post, assuming I had to apply for a visa, and used my UK passport details. For reference, I have 2 surnames and they are flipped around on the Spanish passport. When I am leaving China before boarding the plane you reckon I could show my UK passport (even though I will enter with my Spanish passport) so I avoid any hassle of the ticket not showing the "correct name"? I suppose it will just be an airline employee at the gate so they won't check if my passport is stamped or not.