If you are spending the same amount of time in PVG and HKG, there shouldn't be any price difference in those routings if flying BA longhaul. Both should price either as:

- GLA-PVG return, with or without a stopover in HKG as applicable, or

- GLA-HKG return, with or without a stopover in PVG as applicable

As far as I'm aware, BA's Asia fares don't restrict the stopover to one direction. However the dates you're travelling may affect the price of the fare components. So for example there may be a cheap GLA-PVG fare which allows stopovers in either direction but whose date restrictions mean that it only works if you do the stopover on the inbound, or vv.

By contrast many CX fares do only allow stopover on the inbound.