72 hour transit visa

Bhav, if your itinerary is:

UK - Shanghai - UK

Then you are not eligible for any kind of visa free transit and such an itinerary requires a Chinese visa obtained in advance. Also, it is 144 and not 72 hour visa free transit in Shanghai area, already for years and also this is not a transit visa, but visa free transit, so there is no visa of any kind at all.

The answer to your second questions is, yes, you have to fly to a third country after Shanghai if you want to enjoy 144 hour visa free transit. To simplify, the immediate points before and after your mainland Chinese destination have to be in two different countries or eligible territories (eligible territories: Hong Kong, Taiwan and Macau). In other words, if you create and itinerary to look like:

UK - Shanghai - Dubai

or

UK - Shanghai - Paris

Then you can enjoy 144 hour visa free transit, but you should always have in mind that the immediate points before and after Shanghai are taken into consideration!