I can follow the logic of protecting the stairs from first floor to
loft for all the reasons described, but what is the logic of doing
things to the ground to first floor stairs?

If there is a fire on the ground floor, say a dropped cigarette or burnt out
candle in an open plan lounge, the hallways and stairs must not be
compromised by having those stairs go through the area. The lounge occupants
can flee, the door closes behind (autoclosing) and the loft occupants can
still descend the stairs and can still get out of the front door, despite
the raging inferno in the lounge behind the stairway panelling.

If the ground floor stairs are open plan to the room, then the fire will
disable the entire staircase rapidly. Smoke will rise, even preventing
movement upstairs.

Apparently very few fires happen in stairs and hallways (arson being an
exception). You need to protect the stairs and hallways from bedrooms,
lounges and kitchens, where the fires start. You need to protect the ground
floor stairs, as the stairs are pretty useless as an escape route if they
are on fire (unless you have a second set, of course, which is why 2
independent unprotected main escape routes are permitted).

The general principle is that it is better to bolt out of the front door
than huddle together in the loft waiting for help that might come too late,
because someone ruined the 30min resistant ceiling with halogen downlighters
and the firemen are on strike.

Christian.