Apparently it is not against the law to grow a hedge over 2 Metres unless a complaint is submitted to the local council. Different councils have different laws. You have to have shown you have consulted your neighbour first.
I believe there is a rule about the height of fences (2m) but plants are another matter. Trees grown as hedges are frequently over 10m and as you know there have been many court cases about them. As shenal says it's down to the council.