There was a lot of movement from the countryside into the metropolis - I have a family born and bred in rural Norfolk who all migrated to London around this time.
Any clues to parents in his childrens' names. A John or an Ann for instance?
Any known siblings for Augustine so you could look for their origins - maybe a birth in Norfolk.
Pheno
Added: try the reverse and look at Browns born to John and Ann in Buxton, Norfolk around the same tme and see if any of them ended up in London.