Immigration control in Dublin or your bare assertion of ridiculousness are neither here nor there. Let's get it down to cold hard facts.

Passengers in the domestic/Ireland pier in LHR can walk out the door without hindrance. Passengers who are in transit at LHR and haven't cleared immigration must therefore be prevented from accessing that pier. They do this by photographing people entering from landside who are on a domestic/ROI flight and comparing that against the person presenting him/herself at the checkpoint before he/she can enter the domestic/Ireland pier. Those in transit to a domestic/ROI flight must clear immigration and be photographed before the transit security checkpoint.

A similar argument applies in T5 except that the passengers can't just walk out, but can fly up to MAN/NCL/wherever and walk out there.

The LGW South terminal situation on arrivals is a gaping hole. Just because it is done wrongly there doesn't mean it should also be done wrongly at LHR.

Finally, the Club Europe product includes access to a lounge where available. It does not say which lounge. The wishy-washy posts about oneworld agreements and what-not are irrelevant as they almost certainly cannot be enforced by a passenger as against BA. Taking your argument to its conclusion, can I sue BA for not letting me into T5 Galleries when I'm departing for DUB?

Much of the Trade Descriptions Act has been repealed.

I do agree that the lounge situation in T1 is crackers but they have made the rules clear and our energy should be focused on trying to get them changed rather than tilting at windmills on FT.