i think 125's are too sluggish for green laning unless you gear them for bottom end, ive had a few green laner style 125's like honda XLR 125, KTM exc 125, Superbyke RMX 125. i had the most fun on the KTM as it was 2 stroke and had plenty of power, the XLR is great for just pottering about on green lanes but hills were its worst enemy, i had to slip the clutch like fook to get it up them.
Also keep a local OS map handy with you just in case you run into any problems with the fuzz and you can show then the law
Little vid of my XLR on a local green lane & 4x4 track.