it's sort of in the middle of everything. it's a very very weak science subject if it is even considered one, and though it involves writing quite a few essays it's hardly an essay subject as you don't have to learn to develop your own view. it kinda trains you up for developing a coherent argument, though. it's the only social sci subject (i think) that requires you to learn stuff about how to design and analyze an experiment, so yeah it's kinda like a hybrid of the three.
what i've heard about its linkages with university subjects (apart from the obvious psy or psy joint degree) is that it is being increasingly important in economics (neuroeconomics, game theory, behavioural economics, happiness economics...) and classical philosophy involves some psychology as well (not so sure about this someone pls correct me if i'm wrong! :P)
in terms of studying it for exams, it's quite easy at least for the AS level. lots of ppl find psy theories interesting so it's not hard to learn and memorize them. studies may become problematic as you have to be good with a lot of names and matching the names to the studies and that to the area you're studying for. but all in all you just have to memorize the stuff and write it out in the exam so it's not really hard.
and not all of it is about maths and biology. the maths is really light and arts student handle it well as well. you don't need to know any technicalities of the math (although it would be helpful) and only 1 unit (5 in AS) requires some biology. in the stress unit biological knowledge of the immune system come handy, although there's not terribly much of an overlap. there's quite some long hard technical names (like adrenocorticotrophic hormone) and if you're a biologist perhaps you'd be better at remembering these? i realize i'm generalizing here