I only have French left now, and I've done:
HL Math, Physics, Chemistry
SL French B, Economics, English A1
All of my highers were a lot of work. English, even at standard level, was arguably even more work- but perhaps it seemed that way as I am generally a more number than word-oriented person. The IB? First few weeks teachers settle you into it and everyone keeps telling you how difficult things are gonna get... In all honesty, HL Math was the hardest subject to do at first- if you want to picture it, it's like being in a jet fighter that is catapulted off an aircraft carrier into flight. Your GCSEs are your aircraft carrier, the jet fighter hurtling at over 200 MPH in a mere few seconds is math in the IB. However, I do have to disagree with a lot of the people here and other forums and teachers too- HL Math is not difficult if you understand math. If you are fine with numbers, if you can visualize (this is the most important part) a line in 3D space or what-not, you will do fine- English will be a bigger workload than Math. I think HL Math gained its reputations as an impossible subject from people who aren't initially "good" in Math but think they still want the reputation of being a higher level math stident.
Physics- again, this requires intuition, but if you feel confident with Physics, you'll be able to handle it. HL Chem has the largest syllabus- it'll be roughly 600 pages of knowledge that you'll have to store in your head before exam time- but in the end, it's intuitive, and anyone can do it.
Economics- easy, but I found the last few months difficult as I'm initially not an economics fan and so I wanted to finish as quickly as possible. English, French-these are languages, so any homework wil be essay homework and reading homework. A lot of analysis in English, you'll read between 10-15 books in your two years, and the last few books you'll write a Paper 2 on- so many people read the books 2-3 times (I only read one of them twice, and think I did very well on paper 2 so even that is not necessary).
THE IB IS WHAT YOU MAKE IT! Study hard from day 1, you'll get into the flow. Put things off for later- you'll get into a habit you'll regret of ever having burdened yourself with once July 6th comes and those who worked hard get their 45s, and those who did not work hard enough tell themselves that whatever they achieved was good enough. Anyone is "good enough" for a 45- and if you get it, the IB will have taught you the fruits of hard work and insight.
IF you are interested in World Politics, go for it. What the hell? You shouldn't be studying subjects because a university needs them, and you shouldn't be going to university because a job needs it. You are doing these things for yourself- and the main thing is to stay confident and happy in life. You want to a 7th subject? **** it, do it and be admired and respected for it. Who cares in a university might (and I hardly hardly HARDLY doubt this) not see that you've a done a whole other subject int he world's hardest pre-university program? You'll have done what you wanted- and at the end of the day, that's what matters.