John Young & Son
Architects active in 1862. Our picture is of John Young, 1797-1877, and the picture source says "Mr Young's eldest son, also called John Young, was to become a successful architect/surveyor in his own right." Incidentally, in 1860, David Mocatta (1806–1882), related to Frederick, was working for them.
Wikipedia page for John Young Snr lists many of his buildings in London and credits Snr's humour for the mice. Also:
Born Suffolk. Specialised in polychromatic brickwork. In 1843 built a home for his family: St Mary's Lodge in Stoke Newington, where he died on 23 March 1877. John Young Jnr carried on the practice and died 1910 in Brentwood, Essex.
St Mary's Lodge is a good house in a classic style, on the corner with Lordship Park and, 2016, looks to be at risk.
2017: And Empress Place in Earls Court, by Young, is also at risk: https://twitter.com/saveEarlsCourt .