You're too late to apply to start in September as far as I know, you have to apply via UCAS before the Jan deadline. Or do you already have an offer to go to Edinburgh in september? If so, what I just said doesn't apply.
With regards to people forming relationships etc, although I can imagine the social work students are quite a close group as they see each other a lot I don't think anyone would be like "get lost, you're not in our group!!" sort of thing as everyone is an adult. If you join societies etc no one will care which year you're in, and as its your first year in Edinburgh you might be eligible for university accommodation, I don't know.