I think it's important to know what to expect.
In Italy and Switzerland we do not have those fancy/horrible speed bumps you have in UK, so the risk is anyhow lower.
I mounted it mainly because of the center lower dual-leaf driveway gate locking system that already twice obliged me to change the sump since it has the same height of the sump oil drain plug.
Ouch! I had gates with a central stop plate - I had it lowered so that low cars wouldn't hit it.
The sump protector is a great bit of kit. But it is a rubbing plate rather than some bullet proof thing. If you are going over uneven ground, with high spots, and assuming you're travelling slowly, the plate will tell you that you've grounded the car - without ruining the engine. You either back off, or you proceed slowly. That's its purpose.
I would say, conservatively, that it has saved my engine a dozen times from oil loss through rubbing the sump plug on uneven ground. It has saved it even more times as it touches pretty much every speed bump.
I don't need a frame mounted device as I am not rallying off-road at high speed. This is for 'normal' use.