The problem is which has been pointed out to you on many occasions is thatfrom the mid 60s onwards the population of Northampton and the surrounding areas grew rapidly(the population increase was C5,000p.a.) with an influx of people from around the country predominently London (most of them moved to London N99 on the Eastern). With them came all sorts of deluded fools who were supporters of London clubs who were playing in the highest division and this family tradition is passed from father to son(apart from one case I know where the lad started to support Spurs just to wind his Hammers fan Dad up).
The latest figures for the population of the town is 224,610 (mid-2019 est.). Now you go on about catchment area, so how far down up the A43 do you go before you hit Kettering land? How far down the M1 before you hit Donkeyland? Birmingham, Coventry and Leicester have a good supporter base in the north and north west of the county
Lieceseter to the left of us Dimones to the right, stuck in the middle with you.
Not sure from what angle you're coming from, but, all I want to see is an established L1 club with ambition of taking the next step up, playing in an exciting stadium that holds 12000 to allow just a tiny 2.5% upto 3% proportion of its vast catchment to attend regularly allowing upto 3000 visitors, space permitting.
I don't care where these fans parents migrated from, be it London, the commonwealth or Ireland like my parents did in 1957, I was born in 1961, my first game in 1966 then regularly from 1970..
Football is entertainment, the Stadium is its theatre and if it looks professional, fans will flock.