Isaac COX burial 38 August 1833, aged 34 - so the baptism found - 3 November 1799 St Mary's Northgate, Kent, seems correct.
Ancestry has the entry for the Marriage at Yate, 19 April 1789 of Thomas COX and Grace BURREL (Grace's surname is rather faint)
? Could be talking Military (Army) - Canterbury Kent was a Garrison
So, checking with the National Archives (Just Google A2a) and searching for Thomas COX, Yate
Reference: WO 97/1094/267
Description: Thomas COX, Born Yate, Gloucestershire, Served in Gloucestershire Militia. Discharged, aged 57
Date: 1788-1825
Held by: The National Archives, Kew.
Legal Status: Public Record
Language: English
Closure Status: Open Document, Open Description
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? There is a baptism at Rangeworthy, Gloucester (about 4 miles from Yate) - of Thomas COX, 25 December 1768, parents: Isaac & Hannah.
Grace COX, buried at Gloucestershire St Mary de Crypt - 11 October 1846 aged 78 c1768 - residence: North Hamlet
Thomas COX, buried at Gloucestershire St Mary de Crypt - 27 July 1842, aged 74
There is a Will for Thomas COX dated 21 December 1842, Pensioner of Gloucester - Will look at Ancestry and see what is says.
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This is possible
1841 Census, Glos St Mary de Lobe
COCKS
Thomas 73 - occupation, given as IND (Independent) - all born in County
Grace 73
Harriet 15 (possibly Harriet born 1825 Sussex)
James 13 (possibly James, christened 2 September 1827)