In the UK, there are no subject specific requirements to study Psychology. However, a stronger background in maths is always useful because statistics is ubiquitous in the subject and you need a good basis to build your knowledge of stats from. Some universities thus require higher attainment in GCSE Maths for such courses, but it's unusual. Similarly a stronger background in science is considered useful, as most such courses are inherently experimental in nature - and normally biology is considered the most desirable, as biological approaches in psychology are heavily considered and form a core part of any accredited course in the UK.
However, if you intend to study in the US it's irrelevant, as they don't care what you study - they only care about your grades (and then, your GPA, in general), test scores, and extracurricular activities (and how you portray these in your essays on application). There are no subject requirements to study any course in the US, beyond having taken and passed a minimum background in typically Maths, English, Science, and options - which will have been covered by your GCSEs and year 9 work.
In general, Maths is only a struggle if you don't work at it continually - unlike riding a bike, you do forget maths, and pretty quickly if you don't use it often. However if you do problems consistently (which you should be doing through school anyway, but do at least a sheet of problems a week, maybe even a couple a day) then it quickly becomes second nature. Of course there are other potential limitations - you can't do something you haven't actually been taught, and if you didn't understand it when you were being taught you normally won't pick it up until you "get" it by doing enough problems or having someone explain it better, and then doing some problems immediately to test your understanding; also if you have e.g. dyscalculia or even dyslexia, it can make it considerably harder for some.
In terms of "forensic" psychology (something I might add, is something of a "fad" and the cornerstones of it, behavioural profiling, has been falling out of favour since the 90s due to it being demonstrated unequivocally as being no better than other methods and potentially leading to false arrests and indictments - modern forensic psychology will essentially be you testifying about one particular psychological issue in court, basically as if you're reading out of a textbook) will just require you to have some background in psychology - ideally, to the PhD level (which yes, will involve quite a bit of statistics).
You're extremely early in your academic career, much less your professional one, so focus less on what a single path requires and more on a) doing things you enjoy, which will naturally lead you to work harder in them, work more in them, and as a result of those two things, do better typically (although not always) and maintaining your options, as you may well find something related to what you thought you wanted to do may be more appealing than the original thing. You may find that, while you were originally interested in forensic psychology, you actually found yourself more interested in just learning about humans generally - and want to pursue anthropology. Or you may find you are more interested in the human brain specifically, and decide to pursue neuroscience. If you just tried to ascribe to one particular path, you may close doors to other things - keeping your options open ensures you can pursue your original path and also others if necesasry/desired.
This is particularly true of the US system, which presupposes almost no background at all, and you normally pick up the necessary content as you go along. Doing some more advanced psychology work earlier won't really mean much in the long run if you do end up in that path - if you get placement for the first psychology class you would take at Harvard by taking A-level Psychology (I'm pretty sure you can't but anyway) but take 20 such classes, it really doesn't matter if you had taken that first one or not. However if taking that option prevented you taking, say, Biology and Chemistry together, then that would limit you from applying to many neuroscience or related bioscience courses in the UK. Thus, you would block yourself in a bit.