Hi @Ken83, my first post on this forum, but many years on others..
The first time I came across the symptoms you describe, it was a friend's V12 Ferrari, annoyed him immensely, and getting parts was a nightmare. It can happen to any car, brakes and solutions are universal. In that case, the cause was sticking caliper pistons, which are pressured by the brake fluid to move the pads against the rotor (disc). When the brake pedal is released, the pistons should be sucked back a few thou' pulling the pads off the rotor. If the piston sticks, the pad can stay just touching the rotor - result, squeal, especially obvious at low speed. Any rotor wear can accentuate this.
The pistons can get out of round from heat, get corroded, and insufficiently lubricated. They may need machining or replacement.
Other related issues are water/air in the brake lines (how long since your brake fluid replaced?).
This work needs specialist repair, dealerships don't have the expertise. When I had my Prius serviced last year, my Toyota dealer (biggest in my country, NZ), had a large van on the forecourt with a bloke busy inside. He was a fulltime mobile brakes serviceman, and the van was crammed with small lathes, and other machining tools. I chatted - turns out he is called in by all the dealerships, who just unbolt the assemblies. This is what you need, but it won't be cheap..
So if you really want to keep and enjoy your car, I recommend you hunt down a specialist brake shop (or two) with a good reputation, and negotiate...
