Typically, as with timid police officers, telephone exchanges seem to travel in pairs.
An inter-war brick building, of a utilitarian brick Classicism, restrained in nature, along with a post-war concrete construction.
These are pragmatic architectural forms, constrained within a typology, and responding to the nature of the technology contained within.
Here is the building of 1934 – BT Archives
In 1961 – when Exchange Street was but a rough track, the area was also known as Blueshop Yard.
The concrete cousin constructed in 1971.
So here we are in 2021 taking a close look at these BT beauties, without further ado.
modernmooch.com 
Interesting, showing the advances in phone roll out and development of technology. It looks as if, in the 1960s, expansion of the phone network necessitated extra floor space, presumably to accommodate the bulky strowger equipment maybe still in use. It seems that an extra floor was added between 1961 and 1971. The original architect and engineer may have provided for this possibility back in 1934. With the further rapid expansion of the system it looks as if an additional entire building was needed, still accommodating old-fashioned electromechanical equipment, electronics maybe having not yet taken over. I would guess that with System X and later electronic developments, in the 1980s/1990s, much less space was needed and that most of the floorspace of these two buildings became redundant.
I like the architectural design of the 1930s building – a blend of the classical pseudo-Georgian style then in vogue for public buildings with simple functionality
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