Why assume they were both still alive in 1841? There is a burial in Amersham on 22 Jun 1835 of James Thompson aged 52, so born c1783, which might possibly be him.
There is a record in the Baptist registers for a Henry Thompson, born 18 Nov 1812, son of James & Elizabeth Thompson of Amersham. This child may possibly be the same person as Joseph Henry Thompson aged 17 (born c1812) who was baptised in Amersham on 15 Jul 1829, the son of James Thompson (blacksmith) & Elizabeth his wife.
If so then the Benjamin Thompson baptised 28 Jul 1830 at Amersham to James (a blacksmith) and Elizabeth Thompson may have been another child to this couple, which leads to:
1841 High Street, Amersham
Elizabeth Thompson 50 Grocer
Benjamin Thompson 10
1851 High St, Amersham
Elizabeth Thompson 63 Blacksmith employing 3 men
Benjamin Thompson 20 son Gardener
**** the next family on the census is Simeon, along with his wife Catherine and daughter Emma****
1861 High Street, Amersham
Sarah Aldridge 65 widow
Henry Thompson 48 (born c1813) widower Blacksmith b Amersham
Benjamin Thompson 30 Blacksmith born Amersham
Henry is presumably Benjamin's older brother, as noted above.
Assuming Elizabeth died between 1851 & 1861 then either of these could be her, although the first one seems the more likely:
THOMPSON, ELIZABETH 65 GRO Reference: 1852 M Quarter in AMERSHAM Volume 03A Page 206
THOMPSON, ELIZABETH 65 GRO Reference: 1854 S Quarter in AMERSHAM Volume 03A Page 275
The image isnt online but buried at Amersham 8 Mar 1852, Elisabeth Thompson aged 65 of Amersham Town
ADDED: When Benjamin Thompson married Betsy Haynes in 1861, Amersham, one of the witnesses was Simeon Thompson, so I strongly suspect they were brothers.