David of Broadway wrote:

How wide are the islands though? Leaving aside the shops on those
platforms, the available space for moving down them is so narrow that
most of the time passengers need both sides to move down, especially if
you're trying to get round a buggy. And how exactly would you construct a
TfL users only sealed route from the eastbound Central Line to the DLR
platform?

Fairly wide. These should give you a rough idea:

http://www.flickr.com/photos/triborough/92363857/
http://www.nycsubway.org/perl/show?22191

If I read those images correctly, that's much wider than the Stratford
platforms. No way could a barrier setup like that work there.