UPDATE:
I went into SJSC. today - they are my LBS as well as being a major Brompton dealer.
They told me that the quick release wouldn't fit on my Brooks B17 which seemed odd, but, I ended up buying a zipped roll which will be useful for tools etc. As the quick release and a Barley would have cost more than the Zipped Roll, then I assume he knew what he was talking about. I may get one of the above racks in future if I can be sure that it'll fit and then use the zipped roll on the handlebars.
I need to ring Carradice to ask about re-stitching my Kelbrook bag which got damaged in Latvia a few years back, so I'll ask them about the quick release issue.