TLDR; Wider range of modules you can choose from, no you will not be excluded, everyone else is part of their own department and 'main uni' is the Student's Union which you will be a part of. About 30%
With Economics and Business, I get to choose from a wider range of optional modules. The optional modules for Pure Econ are still quite finance/Econ related (such as Finance for Economists) whereas E&B at SSEES gives me the option to pick up modules in Politics, International Relations, Sociology, East European languages, Film studies, etc. And in 2nd year, you get to choose optional modules from any department at UCL which you can't do with Pure Econ. Personally, I prefer studying a wide range of disciplines rather than just one, so that if I get tired of revising for Economics, I can switch to revising Politics or Business.
And yes, it is a small cohort (around 90 vs 300) but your lectures and tutorials will be spread out around UCL Campus (I only have 1 class in the SSEES building this year) so you won't feel excluded from it. SSEES does host events such as talks but I'm not really interested in them so rarely ever go haha. There's also a SSEES ball coming up some time next year so that is one event I will be attending.
Furthermore, everyone is part of their own course's department so it's not really possible to be exclude from the main uni. I would say that the 'main uni' is the Student's Union which everyone is a part of and they host lots of events so you can meet people from other years doing other courses (
https://studentsunionucl.org/whats-on?s=2022-11-20&e=2022-11-29 for an idea). There's also a really wide range of societies you can join and they host social events. Through living in UCL halls, you will also be able to make friends with loads of people studying different courses so you could always tag along to their events and vice versa.
As for female to male ratio on my course... I would first have to go on a tangent and speak about the Chinese international vs locals.... there's about 70 Chinese and 20 local student (got bored during the E&B welcome and my mate and I counted) in my cohort. I can't speak for the F to M ratio for the Chinese students but from observation I want to say about 35% of them are girls. But for local students, there's about 5 females and about 15 males. But don't be put off by the digits. In your lectures and tutorials, it will not solely be E&B students as other SSEES students will be doing the E&B compulsory modules as their optionals. And you share Economics (200 people in my lecture) with History, Pol and Economics which has a bigger number of students enrolled. My girl-friends in most of my modules comes from the HPE course

Sorry if this was too long, I try to be as informative as possible as this is information I wish I knew while applying!