As she states in every census that she is from Biggleswade I assume this means she really did come from there and not an outlying village - would this be reasonable?
I don't think you can make this assumption, particularly as she was living in Lincs in all censuses. Biggleswade baptisms up to 1875 have been extracted onto the IGI. There's only one Ashwell prior to 1845, and that was in 1785. The same with burials in Biggleswade - there's only one Ashwell,
James John age 15 in 1831. So it doesn't look as though there were too many Ashwells in Biggleswade. Which makes it seem more likely that she may have been from one of the villages near Biggleswade. The most likely baptism is in Langford, adjoining Biggleswade, on 9 July 1820, daughter of James and Mary. Proving it's your Mary might be more difficult!
Do the witnesses at her wedding provide a clue?
I can't see a baptism for the 15 year old James* either. Might he have been Mary's brother?
David
* Probably because his name was John, not James!