Originally Posted by corporate-wage-slave

OK, just can I confirm this, since I've never heard of this happening at T5 before: you were late into LHR, you nevertheless made it through security (via landside or Flight Connections airside? Not that this is relevant to your question), go to presumably a T5C gate, you weren't the first to board, you weren't the last, and at the gate they offloaded you? If this scenario is correct then you have a clear and pretty much unarguable case for Involuntary Denied Boarding, with CJEU precedents to back you. If you were not allowed airside into T5 then it would be another story.