You haven't said what you want to do at uni. . .
Anyway, if you get a B at maths gcse then it may prove a little difficult with the difference in difficulty at AS-level. If you get an A then you do probably have the ability to do at least AS-level. I don't do psychology but from what I can gather it isn't a dos subject as such, but is less intense than for example, maths (please no psychology people shoot me!). I would do maths, but then again, I do maths.