The example on the left is from 1924, the one on the right is from 1935. Both were made from cast iron, and both were designed for the GPO by Giles Gilbert Scott (architect of Liverpool Cathedral and Battersea Power Station).
The later design was smaller to reduce the manufacturing costs and to make it easier to transport, and it's the model you're most likely to see surviving now. Both rather handsome in that picture I think.
I just visited London and spent a large amount of time pointing excitedly and phone boxes, double decker buses and post boxes to the chagrin of my Londoner friends. A great, iconic piece of design, and lovely photos