I am confused by the discussion here of BA’s eligibility-to-enter checks, on the original outbound short-haul flight.

I often travel hand-baggage only, departing London.

I know that when travelling long-haul, I must always attend the passport check desk first, where my eligibility to enter through the border at my destination is checked.

But on short-haul from London, this check does not seem to be required, and I never undergo it. On more than one one sad occasion, this has caused me to discover only at the gate that I have forgotten my passport altogether, when I looked for it to prove my identity (though not border entry eligibility) whilst boarding.

What is the original eligibility-to-enter check here, that the OP claims BA has failed to correctly perform? I am not sure this is actually a check that is done at all, is it? Or perhaps it somehow happens behind the scenes.

T