Glad I'm not the only one confused about this. I have been before on a single entry visitor and know the usual process and can usually turn it around in a week or so if needed.
My understanding is that I'm eligible for the 144. From the UK, have an onward ticket within the 144 hour period. The 144 hours is specific to Beijing and a few other areas so can travel under this visa exclusion.
Seems not many do this then. I'll keep digging.