The queues are horrendous at the 8-11 barrier line, they can't cope with peak-time loadings, I don't see how barrier lines elsewhere would be any better. It'll take 15 minutes for everyone off an 11-coach Pendolino to get out of the station.
I don't find the 8-11 barriers in their present wider form much of an issue. People do pass through them at only slightly less than the rate they can go up the ramp, and not fully clogging the ramp does mean it is just about possible to travel *from* Euston against that flow - previously getting down the ramp was near impossible.
The one big thing I would change there is to have the staff standing by one of the wide gates at the end and adding a clear ASSISTANCE HERE sign. It isn't obvious at present and that causes blockages. It would also help if they would replace the gates with compressed air powered ones that move quicker - I often get caught by the slowness of the current ones.
It'd be much better to have one really long barrier line, but it won't happen because of teh layout of Euston. It isn't just the retail that would be behind the barrier line; the waiting room, toilets and left luggage all would be too. You'd also have to seal off the side gate and side door from 16-18.
The waiting room could be moved, and as the coffee area is being redeveloped at present a separate access to the toilets could be provided. The Sloe Bar has already closed, presumably so that area can be widened. Moving the waiting room and left luggage would not be difficult, neither is particularly big, and in any case might there be a security/personal safety of the waiting room being accessible to passengers only?
There is the rest of the retail, but as more is added outside the station and will be on the new mezzanine I don't think making a few takeaway outlets only accessible to passengers would be a massive problem. It'd only make it like the old arrangement at New St used to be.
I suspect the very narrow access to/from 16-18 might be another reason that is not being done. When they closed the side gate for security during the Olympics, it was a nightmare, and it was reopened as soon as the Games finished.