#OTD in 1854.
Wilkie Collins's "The Lawyer's Story of a Stolen Letter", published as "The Fourth Poor Traveller" in The Seven Poor Travellers – the Household Words special Christmas number – is the first non-police detective fiction published in Britain.
https://en.wikipedia.org/wiki/Wilkie_Collins
The Lawyer's Story of a Stolen Letter at PG:
https://www.gutenberg.org/ebooks/1626