Is uk to Shanghai -5days then on to Japan for a week then fly back to uk with a transfer in Shanghai

Your itinerary, under the condition that those are your all stops and that you are a citizen of 53 eligible countries is definitely eligible for 144 hour visa free transit during your first stop in Shanghai and for 24 hour visa free transit during your second stop in the same city. In other words, you don't need any kind of visa.