Just found the bit he/she/it ? (conveniently) left out.
In the letter he has quoted paragraph 12(2) of the Terrorism Act. He seems to have omitted paragraph 12(1) which states:-
(1) This paragraph applies to a journey-
(a) to Great Britain from the Republic of Ireland, Northern Ireland or any of the Islands,
(b) from Great Britain to any of those places,
(c) to Northern Ireland from Great Britain, the Republic of Ireland or any of the Islands, or
(d) from Northern Ireland to any of those places.
Write to the Chief Constable and report him for attempting to abuse his powers.