I work in a warehouse with offices attached. We have a sweeping plan in place because the sign in/ out procedure was not being used properly 100% of the time. Office workers now work from home but have recently started returning to the office on an adhoc basis. If an office worker stays late at the office, lone working , I cannot utilise the sweeping plan because the this would mean staff in the warehouse would have to leave the warehouse to reach the office and this would take time. I’m not sure how I can safely protect the safety of the lone worker should the fire alarm sound.

They will have to take individual responsibility as you can't expect workers from another part of the site to enter a building that may be the source of the fire, they have a legal obligation to cooperate with measures provided for their health & safety. As long as you have a suitable lone worker system (these days signing in on apps has supplemented paper sign in books and in/out boards) then they have to play their part.