Could you please give me instructions how to find my connecting flight to London Gatwick (CA851 leaves at 0135) after my arrival in Beijing at 2305 on the CA178 from Shanghai?

As the flights are at the same terminal, things will be easy enough.
Transfer from domstic to international. Follow the process:
1:Flight Arrival & Transfer Procedures
2:Inspection & Quarantine
3:Exit and Entry Frontier Inspection
4:Security Check
5:Customs inspection
6:Waiting & Boarding
If you have any problem, just go and consult at the enquiry desk.