1861 census shows that his wife and his eldest child, William, were born in Ireland! Interesting occupation 'traveller with pictures'. Address 122 Porter Street, 'print shop'. He was most likely a travelling photographer/itinerant photographer - you might be able to find his marriage in Irish records?
Do you know (ie. from where Frederick John got married, etc) if the family were Church of England, Catholic, or otherwise?
There's a second possible child for William Lucas and Mary Haig - Edward Marshall Lucas - on the IGI, also at Holy Trinity Clapham.