There is a family of William & Ann GRAHAM in Carlisle who possibly could be the correct parents for your Thomas. It would imply that they moved from Wigton to Carlisle sometime after Thomas's birth. However, I am not sure why they should have been in Wigton in the first place - this William was born in Hexham, and Ann in Carlisle. Anyway, here they are in the censuses:
1841 census for Township of Botchergate, Carlisle
HO107, Piece 175, Book 5, f. 10, p. 14
Wm. GRAHAM, age 50, BkSmith
Thomas GRAHAM, age 15, Cotton Spinner
Walton GRAHAM, age 12, Factory
Joseph GRAHAM, age 9
George GRAHAM, age 4
Ann GRAHAM, age 50
Margaret GRAHAM, age 6
all born in Cumberland
1851 census for Botchergate Township, Carlisle
HO107, Piece 2429, f. 241, p. 38-39, sch. 180
Address: George Sowerbys Lane
William GRAHAM, Head, married, age 65, Blacksmith, born Northumberland, Hexham
Ann GRAHAM, wife, married, age 55, born Carlisle, Cumberland
Walton GRAHAM, son, unmarried, age 22, Ag. Labourer, born Carlisle, Cumberland
Joseph GRAHAM, son, unmarried, age 19, Blacksmith, born Carlisle, Cumberland
George GRAHAM, son, unmarried, age 14, Ag. Labourer, born Carlisle, Cumberland
Margaret GRAHAM, daughter, unmarried, age 17, born Carlisle, Cumberland
Note that their son Thomas does no longer live with his parents in 1851.
I looked for baptisms of this couple's children, first of all in Wigton - but no guarantee that this is actually the same couple. Mary and Ann GRAHAM had baptised in Wigton: William 25 Dec 1818, John 04 Feb 1821, Thomas 18 Jun 1824. Then no more for parents of those names until 1835 (and that one did not match with the children of the Carlisle couple). All the ones born in Carlisle, who are on the censuses, do not appear on FamilySearch as having been baptised. Now I wonder, if this was indeed the right family, whether they had their children baptised in a non-conformist chapel - particularly as Thomas was married in the Presbyterian Chapel.
But as I said, I was just struck by finding a couple called William & Ann GRAHAM with a son of the right age (and the right occupation!), and William being a blacksmith as well. And Thomas married in Carlisle, which also fits.
I think the best way forward will be to get a Scottish birth certificate for one of the children born in Scotland to confirm Isabella's maiden name and the marriage date, then get their marriage certificate.