Can I use 144 hour visa free if I’m flying London to Shanghai, to Paris?

Yes, the 144 hr free transit can cover your trip. No need to worry.

Hello thanks for your reply. Sorry to ask you again, but will this definitely be allowed? Obviously the Chinese immigration officer will know that I don’t intend to use China as a transit through when London is only a few hours drive from Paris? Is this still allowed.

They won't even bat an eye really. Any route is fine as long as your destinations immediately before and after your transit city are located in different countries.