I think I almost understand the layout. If I look at the diagram in the TB you have an additional wall running vertically on plan just to the right of the exit at the top of the plan, and this wall has a 4m gap opening into the larger space on the left. Also, if I understand correctly, the two large glass doors are near this 4m gap so cannot be treated as a separate exit route from the space.
Now a room with only one way out is restricted to 60 people of course, but this space might be interpreted as a room in its own right or simply as part of the larger space. If a room in its own right then it should be restricted to 60 (note, a fire engineer might look to increase this figure slightly because of the generous width of the escape route) and if it is not a room in its own right but simply part of the larger space then there need be no localised restriction on numbers.
The site visit should yield enough information to make a judgement on the space. It's not uncommon for layouts to produce areas within larger spaces that can be interpreted in two ways like this. The people making the decision should bear in mind the uses the space might be put to, the maximum expected occupancy of the space, the worst case fire loading (size and location), the ease with which people can get to and pass through the gap, ceiling height (of course), the positions of tables and chairs and other potential obstructions (if present), etc.
I wouldn't be surprised if the judgement came out that it's all part of the larger space and that there is no problem but I have to say that, like you, I'm not too keen on the idea of having 300 people all of whom are reliant on a single escape route. It seems a little vulnerable to me. We all know that fire safety is about having layers of safety, if we lose one element of safety then there should be others to maintain an adequate level of safety. With total reliance on a single escape route there's no resilience, no margin for error.
If they seem to be thinking it's alright and you're not happy, you can throw in a 'what if' scenario. It might be reasonably foreseeable, for example, that someone sets up a display or coat stand or lighting rig near the 4m gap. What if that catches fire? Where does everyone go then?
