I agree with you about the subjects - the more alike they are, the easier it is. Have you thought about what you want to study at uni OP?
Sixth forms really don't like it - something that I forgot to mention (oops
) was that my school had a sixth form, so everybody already knew me, and I had to have a meeting with the director of studies and all that to discuss my A level choices. I was only allowed because of my GCSE Y10 mock grades (the meeting was in autumn year 11), and I actually had to find enough evidence and stuff to prove that I would survive... All of my teachers got asked their opinions.
That's something that most people don't mention (as you already get enough hate for not doing 3) and talking about all of your difficulties in convincing the school to make a double exception will lead to someone punching you (please don't punch me!)
If you want to do more, go for it. It's your life and nobody can tell you what to do. If you really want to do them, you'll find time and will get the grades.
A level maths and further maths is great. My school does all of maths, then all of further maths (so it's not directly alongside), however, lots of other schools do both at once? If you love maths, it's amazing. If you don't...
Also, if you struggle, there's no shame in dropping one A level (my school said that if I got below a certain mark in end of topic tests for a subject I'd have to drop that subject). If you realise that in year 12, you'll never reach the point where you're getting Bs in all of them.
EDIT: Also, just a tip - always stay at least a few months ahead of the school work if possible. Don't let yourself fall behind or even stay at the place where you should be as then your timetable will kill you (my school only has time in the timetable for 4 subjects, so I'm doing FM on reduced hours. The quantity of compulsory meetings with the head of maths at my school...) If you're a few weeks ahead of where you should be, then all of your classes will be very easy and your homework will take much less time. Also, it really helps to know your teachers, so you should definitely ask them for help/book recommendations/university advice if you need it!