✔ 最佳答案
Since the UK and France are also the member of the European Union, and you have to handle all tax refund in the last stop inside Europe Union, all tax refund application should be done in Heathrow Airport before you come back to Hong Kong.
You cannot, and don't need to do it in Paris.
Be reminded that usually there are lots of people queuing up in Heathrow to handle tax refund procedure, please make sure you have enough time to do it, i.e. arriving Heathrow at least 3 hours before departure.