If fire breaks out in loung at groundfloor level and main stairs catch
fire,
firealarm will sound in loft as they are all connected. Lobby at bottom of
loft stairs has fire door to main landing which is now on fire, so I open
other door in lobby that goes into 1st floor room with window as escape
route, without having to go near the fire.

You need a protected route that doesn't involve jumping out of windows. The
route you suggest would probably be permitted if the 1st floor room's window
is swapped for a door and steps down to ground level.

Having an accessible escape window is the secondary backup. You need to
consider the elderly, infirm and children. Just because you can vault out of
a 1.2m high window to ground 3m below, doesn't mean everyone else can, or
that you could drop your 3 young children unaided out of it. The primary
route mustn't involve gymnastics, even if the secondary route may.

Christian.