Pretty sure it's flat - the concourse immediately after the barriers and platform 8 are level. A short waist-height barrier projecting from the building might be useful to avoid collisions when cornering (i.e. people would emerge in sight before you walking into them), but we managed to deal with this potential catastrophe before the LNER booth arrived...
I've thought about this in the past, and having had a wander around there definately is a significant change in levels. Attached picture is before the gateline was moved (and from Google maps) but you get the idea.
Given there are various historic structures under the platform (isn't this about where one of the sets of toilets in the old subway would be?) I'm not sure it is obvious whether you could flatten it all, or whether you'd have to raise the level elsewhere to overcome the issue - but I would be astounded if it was not possible (at whatever cost...) to overcome this and provie a level area.