Is Hounslow in London or Outer London?

Hounslow is considered part of Outer London.

The London Borough of Hounslow, which inclides the town of Hounslow itself, is defined as an Outer London borough by the Local Government Act 1963, as well as by the Office of National Statistics. Similarly, between 1990 and 2000 when London had separate inner and outer telephone area codes, Hounslow was assigned the Outer London (0181) code.