I don't know much about this, and am yet to visit the Abbey myself, despite walking past it a few times a month, however, checking the web-site, I got the impression that the Abbey was not fully open during the 'lates'... It says "See the highlights of the Abbey at your own pace. Find the Coronation Chair, Henry VII’s Lady Chapel and Poets’ Corner with our Wednesday Lates map."
Highlights suggests limited to me, and Lates Map? why would they need a special map, why not use the normal map... because i suspect some areas will be restricted. But i am guessing.
It's not often you get something for "free", especially for such a popular attraction... but i could be wrong. Worth asking before entry.
I would suspect the queues would be HUGE if the weather isn't to bad, and i would think ticket number will be limited.
Again, I'm guessing.