You could use this box as a free standing box. However, this box includes pre-drilled holes in the back, designed to be securely mounted to a brick wall.

No, the unit does have a sloping lid to help keep rain water out but it is not classed as rainwater proof, only weather and rust resistant. However if you are looking for a waterproof post box I can highly recommend a unit from the Brabantia range Brabantia - B100 Black Post Box

The difference between the Thames and Tamar post boxes is that the Thames post box has a bronze "Newspapers" plaque fitted to the front where as the Tamar does not.