The unis do prefer bio, and it will certainly help you with studying when you actually get there. However, there's no point regretting it now- because no telling how much you would've enjoyed bio, or whether you'd still have managed to achieve the amazing grades you have now.
Additionally, i think they care more about the grades than the subjects they're in - especially for psychology. Maths and psychology are plenty, and will give you are good grounding. On the cambridge website it says 82% of successful applicants take EITHER maths or biology, and 78% psychology, so you're in a pretty good position, and will not be significantly disadvantaged for not taking bio. In a similar position, I tried carrying on with biology as a fourth a level into year thirteen, but the A2 subject matter in just my three subjects was so intense anyway that I decided to drop it in the end. So I recommend focusing on achieving the best possible grades in your other subjects!