Local authorities may interpreted the exempted development schedule differently, but this is what S.I. 600 of 2001 says.
"Class 5
"The construction, erection or alteration, within or bounding the curtilage of a house, of a gate, gateway, railing or wooden fence or a wall of brick, stone, blocks with decorative finish, other concrete blocks or mass concrete.
"Conditions
"1. The height of any such structure shall not exceed 2 metres or, in the case of a wall or fence within or bounding any garden or other space in front of a house, 1.2 metres.
2. Every wall other than a dry or natural stone wall bounding any garden or other space shall be capped and the face of any wall of concrete or concrete block (other than blocks with decorative finish) which will be visible from any road, path or public area, including public open space, shall be rendered or plastered.
3. No such structure shall be a metal palisade or other security fence."
It seems clear that gates up to 2.0M high can be erected without permission to the front of a dwelling, subject to any amendment of the schedule - check this by talking with your planning officer.
Some local authorities take the view that this means that the piers the gate is hung from can also be 2.0M high - piers without gates may not be exempt.
You could seek a Section 5 Declaration in relation to the piers, and submit photographic evidence of any similar height piers in the locality to back up your submission.
ONQ
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
