Originally Posted by
scottishpoet
your problem is you were entering the Common Travel Area.
This isn't the issue. What a person shall do at Immigration is specified -
https://www.legislation.gov.uk/ukpga.../2/paragraph/4
A person on his examination under paragraph 2
[F404, 2A
] or 3 above by an immigration officer
[F406, or on his examination under paragraph 3 above by a designated person, shall, if so required by an immigration officer or designated person
] —
(a)produce either a valid passport with photograph or some other document satisfactorily establishing his identity and nationality or citizenship; ...
This applies to
...any person who has arrived in the United Kingdom by ship or aircraft (including transit passengers, members of the crew and others not seeking to enter the United Kingdom);...
I suppose that, assuming the OP isn't Irish, as a matter of policy, their ID card is not considered as "satisfactorily establishing his identity and nationality". If this isn't made clear on gov.uk it is pretty unfortunate