Witnesses to marriage were William Venville and Mary Rose.

Mary was a "full age" spinster, daughter of William, labourer. Humphrey was a "full age" bachelor. Both were from the Swerford parish.

Interestingly, Banns were read at Swerford in July 1870 for a marriage between Humphrey Knibbs and Hannah Hancox. There is no known marriage for Humphrey and Hannah, so the suspicion is that for one reason or another, it didn't take place.
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The 1881 census shows Humphrey living at Swerford, Oxfordshire, England
Humphry KNIBBS Head M Male 36 Swerford Labourer (Ag)
Eliza KNIBBS Wife M Female 27 Swerford, Oxford, England
Sarah ROSE Daur U Female 7 Swerford, Oxford, England Scholar
Albert KNIBBS Son U Male 4 Scholar
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The 1891 census for Oxfordshire and North Berkshire shows Humphrey living at Swerford, Oxfordshire with his wife and younger son:
Humphry KNIBBS 46 Agricultural Labourer
Eliza 36
Albert 14 Agricultural Labourer

Living with them is William Rose, father-in-law, aged 75 years, a widower and identified as employed as an Agricultural Labourer.
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Humphrey is seen as aged 58 years, and living at West End, Swerford with his wife Eliza (47) at the 1901 census.
Humphry KNIBBS, 58, Stonemasons Labourer, Oxon Swerford
Eliza KNIBBS, 47, Oxon Swerford
William ROSE, Widr, 86, Labourer, Oxon Swerford

Eliza's father was William Rose, a labourer.

It's probably Albert listed in the 1901 census as Alfred, age 24 from Swerford, Oxfordshire, living in Covent Garden, London, working as a Police Constable. There was certainly no Alfred born in Swerford that year or even close to it. He and several other police Constables were listed as living at Bow Street Police Station.
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I discovered from a London Metropolitan Police database that Albert's number was PC 309, Warrant Number 86712. He appears to have joined the Bow Street Division of the Metropolitan Police on 17 September 1900. On 22 July 1902, he transferred to the Hammersmith Division. His number appears to have changed to PC 692 at some point, possibly when he transferred from Bow Street to Hammersmith. He then receives an annual pay rise every year up to 1908. There was a Disciplinary hearing on 10 August 1917 in which he was named, but not sure what, if anything, he'd done wrong.

He was given two more pay rises in August 1918 and September 1922 and eventually, he was Pensioned off on 20 September 1926 aged 49 years:

Date Warrant 1st name Last name Rank Number Division Reason PO Date
17/09/1900 86712 ALBERT KNIBBS PC 309 E JOINED
17/09/1900 86712 ALBERT KNIBBS PC 309 E JOINED
28/09/1901 86712 ALBERT KNIBBS PC E PAY INCREASE
22/07/1902 86712 ALBERT KNIBBS PC 309 E To 773 T 23/07/1902
27/09/1902 86712 ALBERT KNIBBS PC T PAY INCREASE
26/09/1903 86712 ALBERT KNIBBS PC T PAY INCREASE
24/09/1904 86712 ALBERT KNIBBS PC T PAY INCREASE
23/09/1905 86712 ALBERT KNIBBS PC T PAY INCREASE
22/09/1906 86712 ALBERT KNIBBS PC T PAY INCREASE
28/09/1907 86712 ALBERT KNIBBS PC T PAY INCREASE
26/09/1908 86712 ALBERT KNIBBS PC T PAY INCREASE
10/08/1917 86712 ALBERT KNIBBS PC 692 T DISCIPLINARY
24/08/1918 86712 ALBERT KNIBBS PC TA PAY INCREASE
22/09/1922 86712 ALBERT KNIBBS PC T PAY INCREASE
20/09/1926 86712 ALBERT KNIBBS PC 692 T PENSIONED 19/09/1926

We can see from these entries that he was a member of the London Metropolitan Police Force between 1900 and 1926.