Hello, I understand the concept of flying at the maximum legal altitude (120m in UK) is realitve to the elevation of the ground beneath. Go over a hill, the 120m is based on the distance to the ground below.

But I'm intending on flying over cliffs and gorges so when I start at say 30m above ground and then fly over a cliff and I'm suddendly 200m above the point on the ground, what behaviour is the drone going to display? Will it suddendly drop down to maintain 120m above it's current ground point? Will I suddendly be breaking the law by flying over 120m? Can I make the drone fly over 120m so when it returns to me it is above the ground I am standing on, next to the cliff?

Thanks in advance.

The drone will maintain the height,   it wont drop.

If you take off from a clifftop and ascend to 30 metres, then fly out to sea, the drone will maintain the height relative to the takeoff point unless you change it. It will not follow the level of the terrain. Incidentally, take care as you fly over the edge. Even if it doesn't feel particularly windy, the wind shear coming up or down the cliff face can play havoc with the stability of your drone.