EFA I'm only a little surprised that you've used 500mb in 1day though it is quite a lot. I had issues with topping out the 500mb in 14-21 days for the last 6 months of my iPhone 4 contract with O2.
Basically O2 don't know or care where the data goes. iCloud isn't the reason - it will only sync on wifi- it does nothing else unless you are pulling down apps/ music from the cloud actively.
O2 won't/ can't give you any form of helpful itemised bill- data adds itself onto the bill on and off- not when it is being used.
I was told I must be streaming radio of tv off a wifi network- I wasn't. That " apps work in the background and eat data"- yes they do but not hugely though Facebook can be big and delete that and use a safari window for now. I did a careful monitoring I've my use and think it must be flipping work emails with attachments that were the main issue.
Apple advice is take all apps off and monitor as you re add - hmm- but then this is 3rd party stuff do " not their problem". Some forums seem to think when iPhone " wakes up" it will connect and transfer ata over 3G before it connects to wifi- I'm not convinced.
What can you do- well unless you can prove miss selling ( did the definitely say " 500 will be plenty your normal use is x"?? ) you are stuck with the contract I believe.
I have the datman app to monitor data and ovano to shrink it- ovano does help - and absolutely strangles any streaming etc.
Turn off cellular data when you are on wifi - don't give the phone a " choice" and I even turned my phone off during the day at work. Makes an iPhone less functional but decreases data.
Buy a bolt on now so you have some data for the rest of the month but make sure they confirm that you can cancel so t doesn't automatically renew.
MSE mobile forum may have more help for you too.
( me I've kept the 4 at the moment and Am on giffgaff unlimited data to see how things go, I really think iPhones need far more data for most users than they are contracted for)