London - Shanghai - Paris - London for 144 hour free transit, or is there a better way?

I will be visiting Shanghai for under 144 hours. Ideally I would just fly from London to SH and back to London, but one must enter a third country for it to be valid?

Is there a better route to take? (quicker/ cheaper?)

Also, do I need to do anything prior to arriving at the airport in London to arrange the visa free travel?

Thanks