Airport catchments do not really reflect endemic populations, if an airport is convenient and accessible and offers connections, direct or otherwise, they will use it.
Well that's a start but :
1) Price has to be right
2) Frequency matters, if EK/QR/EY can offer a daily one stop for cheaper than a twice weekly non stop route, then the latter will not work, see PIA at Glasgow for proof.
3) There's not enough Indians in Scotland to maker irt viable unless you assume that Pakistanis (there's loads of them in Glasgow) are the same as Indians and they'll happily connect over Mumbai.....think about it ! By your own stats Joe, PIK should be doing way better, once again you choose the stats you fancy and present them in isolation.