Travel question

Only the stops before and after China matter, so your route London - Beijing - London is not an eligible route. It's a return trip, not a transit.

You can make your itinerary eligible by taking a flight from, let's say, Paris to Beijing and then return from Beijing to London. For example:

London - Paris - Beijing - London

This option would be eligible for 144 hour visa free transit, but flights Paris - Beijing and Beijing - London have to be of nonstop type.