IMO MU is the obvious carrier for you to use for flights within China. For destinations elsewhere, MU could make sense or you might prefer partners such as KE. Then the question is whether to continue crediting to DL or to switch to the MU FF program. You'll need to research their requirements as well as the benefits they provide that you wouldn't get from DL status. IIRC they are more generous about granting lounge access to their own elites traveling on MU domestic or international flights.
BTW do you speak Chinese? This would probably make it easier to deal with the MU program.
CZ is an alternative, but there are rumors that CZ might leave SkyTeam. Many CZ connections are through CAN, where CZ is based, versus PVG four MU. Nevertheless, CZ has a reputation for the best customer service of the mainland Chinese carriers.
If you stick with DL, don't forget that partners (not in SkyTeam, at least not yet) VA and VS operate some long haul flights in Asia, including to/from LHR.