I too agree arthuruk. I also have considered either one of them using a different name at marriage OR they never got around to marrying. That said, I haven't found on 1841, 51, 61 census a Thomas Chapman born circa 1840 Willesden (other than the beer house keeper who is accounted for)
I've only had a brief look, but I haven't spotted this beer house keeper at all. Do you have a reference or alternative spelling, please?Thomas Chapman born 1839/1840 Hendon is married to an Elizabeth, I can't remember her surname but they are also still living in Willesden in later census reports, beer house keeper.
My Thomas is very consistant in census 1871-1911 with his birth year 1841 and place of birth Willesden. But there is no other Thomas born in Willesden in 1841. Also in the 1911 census Thomas states that he was married for so many years which worked out to be 1864 and on Elizabeth's death in 1910 says that she was born 1837.
I think it quite strange that I have been unable to find no sign of either Thomas and Elizabeth at all before 1871.
Meanwhile, I did find this Thomas in Willesden in 1841 & 1851, plus his mother's death - since the names match, I'm assuming this was the one baptised 15 Dec 1839:
But I don't think I tried looking for father John in 1861 or later.1841: HO107/690/16 fo34 p14-15
John (grocer) with Margaret (laundress), both "30", plus 5 children including Thomas, 2; also Elizabeth Chapman "70"? 78? - John's mother? Adults not born in county.
1851: HO107/1700 fo388 p30 (actually you've already got this)
John (grocer & general dealer), 44, born Lingfield, widower, plus 6 children including Thomas, 11. Children all born Willesden.
Death of Margaret Eliza Chapman: Dec qtr 1846, Hendon, 3/171
Buried St Mary, Willesden 27 Dec 1846, age 39