Not wanting to be picky...
..especially when it's a Mod but
The ANO defines the heights in metres.
Above 90m AGL certain objects may be obstacles and require lights subject to an aeronautical study.
Above 150m AGL all objects are obstacles and will be lit.
Charting is the answer. If you flight plan then nothing on the ground should come as a surprise.
Any issues with obstacles outside an aerodromes should be referred to the Director of Airspace Policy at CAA House, Lunnon.
Sir George Cayley