I think the term loses a lot of meaning if used just for the same distances in the UK as in the US. Particularly because the main consensus for 'suburb' seems to be that it's a residential area, which the commuter flows suggest that Reading is not. Perhaps the best benchmark is merely whether a place is treated as a sub-district (albeit far-flung) of a city? I'd say that doesn't apply to Reading, but perhaps the same is true of Fairfax.
Your final remark is a little confusing - what is consistent looking at commuter flows? You mentioned that large numbers of people commute from west of Fairfax into DC, and I said Reading has a net influx of commuters. Those statements don't directly contradict each other, but certainly don't explicitly agree!
Suppose 100k people live in fairfax and 150k people work in fairfax. That's a net influx. But if 300k people commute though fairfax IMO that point's to it being part of a larger collective.
Your final remark is a little confusing - what is consistent looking at commuter flows? You mentioned that large numbers of people commute from west of Fairfax into DC, and I said Reading has a net influx of commuters. Those statements don't directly contradict each other, but certainly don't explicitly agree!