It all depends on your mindset, how much you're able to focus, and how willing you are to sit down and do the work. Okay, there's definitely a degree of difference between one person and the next in terms of their innate ability to understand, manipulate and contrast complex ideas, but I reckon a lot comes down to will. At the equivalent to GCSEs in Sweden I had all Cs and one B, and for my A-level equivalent I was on a B average. I took two years out before going to university, and graduated this summer at the top of my year of some 130 students and five prizes for my performance (braggity braggity boast boast)- I was never a particularly good student, but I've never been stupid as such. The difference between me in university and me a few years earlier is one of maturity and dedication - while a lot of my course mates did the bare minimum to get a pass or a B (a lot of people, oddly, seem to aim for a 2.1, which I'm grateful for as it worked in my favour), I'd sit and work my arse off until I felt the essay was as good as I could get it or I knew as much as I was ever going to before an exam.
I know a lot of other people from various universities who achieved high firsts as well, and while they are definitely intelligent people, I also know that they would work up to 14 hours per day even during term (i.e. not revision). Hard work pays, and if you actually like the subject you've opted for, it doesn't feel much like work for most of the time. That said, there were times where I was extremely fed up, but push through the barriers and keep at it and I'll sure you'll do absolutely fine - if not outstandingly well.
EDIT: As for what sort of work to expect, I would imagine psychology to be quite similar to management/marketing (which is what I did). A lot of our concepts come from psychology and I'd spend a lot of time reading psychology journals. Anyway, as it's a social science I would imagine that you will be doing a heck of a lot of reading, and probably a fair old bit of essay writing. I wouldn't be surprised if a majority of your modules are 40-50% essay based, with exams consists of 2-3 essay answers. That's just a guess, though, and reflective of my own experience rather than any knowledge of psychology degrees.