Startup mentality and boys club culture stem from senior management
Pros
Overall you have the ability to talk to senior management directly about concerns and feedback, however secondary level management isn't as mature to take on the feedback without taking it personally. Young office, casual dress all week (personal preference would be for business casual instead), dependant on your role and client base, you have the chance to work with colleagues globally as you would expect. HR has implemented an employee council which empowers staff to feedback to management and implement some changes.
Cons
Company culture is definitely the typical 'sales orientated, drinking every week' culture - if you don't drink to excess you are 'out', this culture is driven by senior UK and global management, company events are (in my experience) all about drinking. HR is still a young function and is trying to implement procedures and process, unfortunately due to the 'boys club' culture, I can see potential for sexual or discrimination grievances unless management changes. Secondary level management varies, and there doesn't appear to be a proper focus on soft skill management training to ensure that you are retaining good staff instead of them leaving due to bad people management.