LHR-CAN ---- CAN-BKK-LHR

Thanks in advance!
Australian booking flights and going to utilise the 72 hour twov.

If i buy a one way direct flight from London to Guangzhou with airline A but my second flight with airline B is Guangzhou to London (with stopover in Bangkok) will that be viewed as an ineligible LHR-CAN-LHR or an eligible LHR-CAN-BKK-LHR?

My understanding for the trip back to London is that it's separate boarding passes so it should be fine. But if not i'll fork out the extra for a direct from CAN-BKK and then BKK-LHR.