Hi there, it seems you've worked yourself into a (little) panic!
I was in a similar situation last year, when choosing between IB and A-levels. First of all, I have to say that the IB is less hard than a lot of people are lead to believe- lets face it the year above is probably trying to scare you. It seems that in your situation- not knowing what subjects to do yet- the IB may be better in terms of keeping your options open. From my experience so far, IB subjects are almost as detailed (or in the case of maths more detailed certainly) as A-Levels, so you will not be missing out on material.
With regards to procrastination- the workload that you will have will ensure that you will simply eventually not be able to spend to much time procrastinating- a good mindset for uni as well.
I am taking geography in a mixed SL/HL class, and I have found it barely a step up from GCSE level. Also, if you are getting A*s in all your geography exams, do not worry, you will be fine. I must say however, that B languages at Higher Level seem to be a big step up (I take Spanish B), but again, with work and time, you will gain more from the course.
I was wondering if there was anyway you would be able to switch to A-levels if you decided that the IB was too much after a few weeks? Surely your decision will not be completely final by September? It may be worth simply trying the IB and then, if necessary, switching to A-levels.
Finally, Unis do realise that the IB is harder (but as I said earlier, definitely NOT impossible), and therefore will often require lower IB grades relative to A-Levels, particularly in Russell Group unis, e.g. Durham.
I hope this helps- I may be slightly biased towards the IB :P