travel Heathrow-shanghai - Hanoi return Ho chi Minh-Shanghai-Heathrow; which visa for 26 hr stopover

If you have mentioned your all stops within your itinerary, you do not need any kind of visa.

Your itinerary is definitely eligible for 144 hour visa free transit in wider Shanghai area twice. All you have to do is to declare in London that you will travel to Hanoi via Shanghai and the same thing on your arrival in Shanghai. When you land in Shanghai you will first approach 144 hour visa free transit counter where you will present them your onward ticket and it is recommended also to present an accommodation booking confirmation. After a short check, you will be allowed to exit the airport. The same thing is valid for your travel back from Vietnam to UK.

Again, you do not need any kind of visa.