Tom, I've found 3 very interesting pieces of information for you.

Firstly, 1848 Marriage Banns for James Kewley:

Copied from: London, England, Marriages and Banns, 1754-1921
Southwark, Southwark St Saviour, 1848, image 50
No 661 Banns of Marriage between James Kewley, Bachlr and Isabella McLatchie, Spinster, Both of this Parish were published on the Three Sundays underwritten:
That is to say, On Sunday, the 26 March S Benson
On Sunday, the 2 April S Benson
On Sunday, the 9 ditto S Benson

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BUT, I can't find any evidence that the marriage took place.
Isabella McLatchie age 30 was living at 8 Millman Row with Arabella Baker in 1841, as related in my last message.

Secondly, I've found the baptismal entry for Louis/Lewis Philip Kewley Kewley:

Ancestry's version in the Baptism index:
London, England, Births and Baptisms, 1813-1906
about Lewis Philip Kewley Mclatchie
Name: Lewis Philip Kewley Mclatchie
Record Type: Baptism
Estimated Birth Date: abt 1848
Baptism Date: 16 Oct 1848
Mother's Name: Isabella Mclatchie
Parish or Poor Law Union: Clerkenwell St James
Borough: Islington

My note:
NB the "estimated birth date" of "abt 1848" is definitely a wrong reading of the image.
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The actual image shows:

Islington, Clerkenwell St James, 1848, image 19

Baptisms solemnised in the Parish of St James, Clerkenwell, in the county of Middlesex, in the year 1848

October 16 / No 816

Child's christian name: Lewis (but could also be read as Louis) Philip Kewley

Mother's name: Isabella Mc Latchie / no father's name given

Address: 8 Millman Row, Chelsea (born 21st December 1839)

Signed: C P Farrar

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This suggests that the marriage hadn't taken place before October 16th 1848, and Isabella decided to have 8 yr old Lewis Philip baptised with her own surname.

I wonder whether Isabella's health was poor and perhaps she died before the 1851 census --- I can't find any trace of her. But James Kewley describes himself as a widower in 1851.

Thirdly, I've found James Kewley b 1783 in the 1841 IOM census, staying with the children of his late sister, Sophia Killey nee Kewley:

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1841
Onchan 8, 17+18
Castle Street

Ancestry's transcription (Kelley should be KIlley ):

Name Age
Philip Kelley 25
William Jackson 15
Susanna Kelley 27
Jane Kelley 20
Marian Jackson 14
James Kewley 50
Marran Kewley 4
Bakarran Kewley 60
Margt Kayll 25
Ann Callow 18

My notes:
James Kewley was a CIVIL ENGINEER (civil eng) b IOM
Marian Kewley 4 was b England
"Bakarran Kewley" 60 looks like Isa Karran (and there are no ditto marks to say that she was a Kewley)

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I have more on this family in subsequent censuses which I'll put into another message.

Jean C