Oh I see. Yes, the orchestra pit is bridged over rather than rafted over, it’s quite a wide bridge which I take to be the main route out towards the concourse, and that bridge has been open and in use since summer 2017, although it had temporary side walls and a temporary roof initially. But going down the stairs at the platform end to the orchestra pit level itself isn’t really a main route to anywhere except the new toilets, yet. There’s also access from the mid platform area down two flights to the LU Bakerloo/Northern level, which corresponds to the P1-19 access to LU via the peak hours subway.
Lastly, and as you already mentioned, there is still the existing ability to get through from P20 to P19 and use the P18/19 barrier line alongside the LU escalators. That route has had a ramp added as well.
The station plan in image 4 of post #119 shows all the various stairs, ramps and lifts, as well as the bridge, but not the dedicated escape stairs towards the country end as the full platform length is not shown.
AIUI there is no specific requirement for “PRM capable” evacuation from the outer end of platforms, based on brand new builds such as Reading or London Bridge. Stairs is all you get.
Without being Waterloo specific!
It all depends upon risk. If the far end of a platform is in free space, so not likely to find it's self engulfed in smoke, then it may be acceptable to the local Fire and Rescue Service to designate that area a refuge area. The idea being that anyone trapped there by, say, a train fire could expect to be rescued by the F&RS within 2 hrs. If there is no chance of getting to an area that is likely to be smoke free then different requirements apply, think London Underground new stations or Crossrail.
Network Rail take their responsibilities very seriously indeed and work well with their own Fire Officers and the local F&RS organisations so I'm sure they will have covered all that is required for Waterloo.