They are in South Stoke in 1841 - Ancestry has them as Carril, and their son John (I assume) and family as Carriel.
The only Balance street I know of was in Walcot, the other side of Bath. John and Emma were married at Walcot church so perhaps it was her family that lived there.
I can't see them anywhere on the CD of North Somerset Marriages, with any variation of Cavill, Carvil, Carril etc. I can think of.
John appears to have been baptised in Hinton Charterhouse (from FreeREG)
Charterhouse St John the Baptist baptised 21 Jan 1821
John CAVELL parents Thomas a lab and Martha CAVELL of Hinton Fathers Occupation Labourer
They also baptised Elizabeth at Hinton in 1813 and Maria and Ann (as Cabel) in 1815 and 1818. Hinton Charterhouse is on the southern edge of the area covered by the North Somerset CD, suggesting you want to look a bit further south for the wedding. It's not far from Wiltshire either, but Thomas and Martha both said they were born in Somerset in 1841.
There's also an Elizabeth baptised by Thomas and Martha Cavel in Publow in 1810. A bit far away but, as we can't find one marriage let alone two, there must be a reasonable chance this is the same couple.
HTH
David