Airport tax is called Air Passenger Duty (APD) and is imposed on passengers by the government. Heathrow is not involved in levying or collecting this tax. It is paid when passengers book their ticket with the airline, and collected by the Government once the journey has taken place.
There are different rates of APD depending on the ticket types and journey type (EU/non-EU). For further information see the HM Revenue & Customs website. Taxes and charges are explained by the Civil Aviation Authority (CAA) website.
For information on airport charges, which are imposed by Heathrow on airlines (not a tax), please see our regulation information page.