Birthplace [21 March 2014]
I don't yet have enough information to determine where Timothy was born. I have a baptism record from 1774, for a Timothy Kelly, son of Timothy Kelly. It is in the same parish where he was married, so it may be him.
I don't know how much use this is, but Geni.com has this for free viewing: Birth: estimated between 1771 and 1831 Son of Timothy Kelly and Mary Kelly Brother of Michael Kelly and John Kelly--Ben 23:24, 21 March 2014 (UTC)
Question of which St James parish?
Contrary to the statement above, the 1774 baptism and the 1808 marriage aren't in the same parish. The images included for each annotate them to show that the 1774 baptism was at St James Clerkenwell, and the 1808 marriage was at St James Picadilly, also known as St James Westminster. Both are in the general London area and even then were part of the built up area, but in very different parts of town - Clerkenwell to the north of London and Picadilly to the west. It's not impossible that it's the same Timothy but, given that this is London we're dealing with, absolutely no guarantee.