We have about 150 staff in our biggest office, over three floors and a lot of meeting rooms. 

I've seen the 'train everyone' argument advanced, buit don't believe it works for fire wardens - if everyone is checking that everyone is evacuating and checking rooms, then no-one is evacuating, they are all sweeping.  I just don't see it working.

For first aiders I think we will need about twice as many as we've previously had.  I think completely flexible whim-based attendance doesn't work foir first aiders (i.e. it simply won't be possible for employees to wake up in the morning and make a decision then whether they will go to the office or not).  If you have a core of people that are in most of the time, you'll simply need to arrange cover for when they aren't in (and if someone is covering an office manager's holiday, they'll need to go into the office).  Your emergency fallback for first aiders is to have some designated individuals - but that would be a last resort if your planned cover is sick or involved in a traffic accident on the way to work or something.

I think fire wardens are a bit more difficult.  We rely on designated individuals sweeping areas, and when the  individual is away someone else covers their area (we have about 25% more 'understudies' trained to provide this extra cover).  You need to schedule holidays, but we require people to book and get approval for holiday anyway - fire wardens simply need to get director approval for their leave and also arrange an understudy.  I think we will need something similar, but with two or maybe even three fire wardens for each area effectively on a rota (so Zippy does Monday and Thursday, Bungle does Tuesday, and George does Wednesday and Friday, or whatever).  I think each fire warden will be getting cheaper training than previously - we used to send them all to actually use an extinguisher on a fire, but if we have three times as many I think maybe even e-learning plus mentoring from an 'old hand'.

Training everyone can work if you combine it with a method for deciding who is "It" on a given day or when the alarm happens to go off.

A method I've used for deciding who is "It" is hanging up a hi-viz vest at a visible point and declaring the first person to pick it up to be "It".

I'm not claiming it was 100% reliable but it's not completely unrealistic with adequate training and communication.

You could even offer a reward for being "It" to incentivise people to do it.