I believe most iDrive cars are prewired for Bluetooth, do you have a 'shark fin' on the roof?
If you are prewired, then you just need the Bluetooth module installing and the car coded by a dealer. Not sure what's involved with iDrive cars if you are not prewired, but it will probably involve either many £'s or many hours.
It does have a shark fin on the roof and Im a little surprised it wasn't equiped with a bluetooth facility.
I've contacted BMW direct and they have said that I need to contact my local dealer, which I'm trying to avoid as I now it's going to cost big time.
They should know 100% yes or no whether it was pre-wired or not. You have to badger them as they don't like working for free, lol.
If it's not pre-wired, then yes it will get expensive. I spent over a month sources parts from various countries to try and get the oem bluetooth look. To be honest, in the end, it was not worth it at all. It was looking to cost £600 to £700 which is crazy.
I have now found a much better solluction which will be great and so much cheaper.