Wouldn't it be cheaper to buy some land and dig a lake?

Possibly, but coming across land on the market with the right potential is very difficult.

The land has to be suitable physically i.e. able to hold water and have the benefit of a suitable water supply. It also has to be in a place where planning permission for the construction of a lake will be granted. The planners will examine issues such as the scale of the project, access, highways and how the proposal fits with stated local objectives. Furthermore, you must obtain consent from the Environment Agency.

Even once you had the land and necessary permission you would still have considerable construction costs, stocking costs and be several years from generating income from the site.