The Historical Directories collection at Leicester University have a few for that era and area online for free
http://specialcollections.le.ac.uk/cdm/map/collection/p16445...as does Ancestry.co.uk - go for UK, City and County Directories on the Card Index
They will not contain lists of staff for a firm and his entry (should there be one) may only list his occupation. There may not be any indication of his employer.
You will need to look under London and Middlesex (depending on what areas constituted 'London' at the time!). St Pancras is in the Borough of Camden.
HTH
Fiona