The brakes on my 2011 XF Premium Portfolio(21K miles) had been squeaking. Just a bit(but loud enough hear well) when lightly applied at low speed and a lot when using light braking to pull the car in the garage.
Over time the dealer has offered a host of reasons why, all coming down to how they think I drive the car because "Jaguar brakes are fantastic" of course(braking too hard, braking too soft, riding the brakes, car sitting too much...etc).
I kept on them and finally yesterday they found a service bulletin JTB00256-Brakes - Squeal From Front Brakes 355mm Discs Only. There is a replacement part required for the front caliper springs. I just picked the car up this morning after having the springs replaced and it doesn't make a peep! It now functions as it should have originally.
Hope this helps if your having a similar issue.

 

I have something similar with only 18k miles. I wonder if I'll get away with just this part. thanks for the post.

 

Mine ended up having the same issue. Caliper spring changed and problem fixed.