Pros
The flat structure was attractive for a non-departmental public body (NDPB), where it is usually quite hierarchal. Regardless of seniority and team, everyone sat next to each other in the open plan office. Sometimes you'd end up sitting next to the CEO or Senior Directors which may appear scary at first, but was actually a great way to build relationships informally.
The pay was also generous for interesting work in the public sector sphere, where pay is usually quite low despite the cost of living in London.
Cons
The office felt at times too small to house the number of London-based employees, especially on popular team days such as Tuesdays and Thursdays. It was difficult securing bookings for the correct sized rooms, so it meant sometimes you would have to join team meetings at your desk (which felt a bit counter intuitive after commuting in).
Information record management wasn't very sophisticated, and you had to rely on relationships to find important and key documents for your work. Better information governance training could be implemented to promote a culture of properly using information management systems.