As far as I remember Air Passenger Duty is based on the entire journey including connecting flights if the connection is made within 24 hours.
Ways around paying are to buy separate non connecting tickets, have a stop over on the way or use an alternative such as Eurostar or Ferries to get out of the UK and then fly onwards.
Open jaw tickets are a worth considering as prices can be similar to or less than a normal return. Eg Paris - Your Destination - London.
Travel search engines should give you an idea of what works.
Air Passenger Duty (APD) - ABTA