Mystery Pipe in the Garden (UK)
it's difficult to determine without a scale reference, but if I found this sort of thing in my yard, my first inclination would be to tap on it to determine that it's a solid rod and the suspect cap is a clamp used to attach grounding cables. The shape of the top of the rod/pipe appears to be the result of hammering the rod into the earth. In the USA, one can expect an 8 foot (>2 meter) long rod to meet specifications of many areas of the country.