That might help, especially if arriving passengers were funnelled towards the centre of the existing concourse and down the first two escalators (which are permanently downwards today, apparently because people kept stopping at the top to read the departures screens and other people would then collide with them). However if the departing barrier line was brought back to alongside these escalators, people waiting for trains to be announced would do so between the opening to the shops below and the side wall or the Eurostar fence, both of which are rather narrow.
Permissive working is used at least once in most hours, but obviously isn't a long-term solution if train lengths increase. However timetables could be changed to reduce platform occupancy. For example the Nottingham train arrives about xx13 at present and leaves at xx15 an hour and a bit later, so one is always in the platform in the off-peak. It has about 35min in Nottingham so if it was speeded up slightly and the timetable re-jigged it could have say a 90min journey and 30min each end, needing one train fewer than it does today. However, thanks to Mr Grayling, that acceleration and cost saving may not be possible with bi-modes.