travel back to Uk via Shanghai

Hello I plan to travel
LON-BKK ON ONE FLIGHT TICKET
THEN THE RETURN LEG IS

BKK-SHANGHAI (air asia)
SHANGHAI-LONDON FINAL DESTINATION BA (2 days in Shanghai.)

Can I use the 72 hour visa, because will be travelling from a third country (Thailand) to Shanghai with an onward ticket to UK and less than 72hours ? In other words does the policy work for flights going back to an allowed country coming from a different destination. thanks