With not much knowledge of London areas (apart from a few), I found this which looks a good fit according to yr of birth to have had a son bap. 1805
1851
John MacDonnell
Marital status Widower
Age 70
Birth year 1781
Occupation Private Soldier Royal Hospital Chelsea
Birth place Scotland
Parish Chelsea
County Middlesex
Parliamentary borough Chas Saint Badcocke
Registration district Chelsea
Archive reference HO107
Piece number 1472
Folio 551
Page 29
Note though, as he was in hospital someone else would have given the spelling.
I don't see him in 1861 on a census so looking for a death 1851 - 1861 may hold a clue but that's a BIG ? as he was a widower & probably his death was registered by a hospital employee although maybe not....may have been his son, hopefully?
I wonder if this birth is by the same John & Eleanor....
Alice Elizabeth Ann Macdonald b 13 Jul 1807 & Christened 11 Oct 1807 at St. James, Westminster, Middlesex, England to John Macdonald & Eleanor....don't you wish they would have used maiden names
https://familysearch.org/ark:/61903/1:1:NDWJ-Q4MAdding this as the names John & William are included & may be of use
1841
Barretts Court, St Marylebone, Marylebone, Middlesex, England
William McDonald Male 45 1796 Ireland (Occ. Boot M)
Probably MAKER Ann McDonald Female 39 1802 Ireland
Jerermia McDonald Male 12 1829 Middlesex, England
John McDonald Male 10 1831 Middlesex, England
Maria McDonald Female 8 1833 Middlesex, England
William McDonald Male 1 1840 Middlesex, England
Annie