The exception is that the airport is not a part of the UK sovereign territory, but is rather considered an extra-territorial possession of the UK, akin to a colony, which can be governed separately.
Thank you. I guess that could be the explanation. Still, it's strange that you could be "detained" and interrogated and not be allowed to see your lawyer, and not have the right to remain silent. In the US I would expect to retain all my rights even if I travel to the US Virgin Islands, for example, even if these islands are not part of any state.
AFAIK, the same is true of the USA: its international airports are extra-territorial possessions in which the USA is not obligated to enforce its human rights agreements ...
