Not sure if it is still the case but Planning notice 24 stated
A helicopter landing in a field does not necessarily constitute a change of use unless a hard surface, hangar and landing lights have been erected.
Also if helicopters have been landing there for 4 years ( might be 10 not sure ) then you can apply for a letter of lawfulness providing there have been no issues and there is nothing the council can do.
Why do you want planning permission, only really need it of the council starts complaining !