Hi I plan to fly: London-Shanghai-London my trip lasts less than 72hrs do I need to apply for a visa

Well, in this case, you need to get an entry permit because you fly in from London and fly out to London. The route is not considered as a transit route. I advise you to fly from London to HK first and then fly to Shanghai. After your trip, fly back to London so that you can make use of the 72-hour free transit policy.