"however well 6FC actually do in terms of outcomes) whilst pretty much all of the lower attainers go to college, it seems that on a national scale and taking the country as a whole (instead of focusing on individual areas which often don’t have school 6th forms anyway) the assertion is actually broadly right still??"
I honestly don't think you can make any national generalisations, because the local picture is too varied.
In areas where everyone leaves school at 16 (Hampshire for example) most bright students will go onto college.
In other areas, it's harder to get into some elite sixth form colleges than it is to stay on at the school sixth form (hills road being an example, although you'd have to be outside of the CAP)
Some areas are now developing the elite private sixth form colleges... so again, these can take from other independent schools.
Where I live, it depends on the school. For example, at my small school sixth form all the brighter ones left (mostly to sixth form college, some to other sixth forms), the weakest got apprenticeships or went into FE, and the weaker ones stayed on at the sixth form.
I don't think you can make a generalisation as the national picture is too mixed, as have been seen on this thread.