So I pondered if it is in some way connected to a similar building in Upper Street, N1, which also features faces within roundels that are extremely similar:
Perhaps it was designed by the same architect or, more likely seeing as there are many terracotta buildings across London, it was just a standard style and these portrait reliefs, roundels and and other stick-on details were readily available to buy off-the-shelf from the local architectural mouldings emporium...?