I bet like me you drive hard and therefore brake hard! If so, it's not unusual to get squeels at low speeds as you will have polished the surfaces of your pads and disks like mirrors.
Also, some pads just squeel - I have had several sets on my e39 already, some have squeeled all the time (after a couple of months bedding in) and others just at the end of their life.
yOU USED TO GET "ANTI SQUEEL SHIMS" with every set of pads you bought (at least over here you did), which were a thin peice of tin which sat between your pads and the caliper - they really did work. Seems you just don't get these any more.
If you don't mind getting your hands dirty, remove the pads and liberally coat the back of them with "COPPER GREASE", this usually works as well as anti squeel shims.
If you don't like to get you hands dirty and BMW can't find a problem there's not much you can do other than put up with it or pay for some replacement pads and hope they are better.
Just on the side, I know how bad it feels to pull up in a heavily pedestrianised area in what's supposed to be a luxury saloon only to have heads turn not to admire you ride but in distaint at the horrible high pitched squeel it emits as you stop.