When I had driven some 150 miles a sharp, metallic tone started to come from the front. First I thought it was the motor but pressing brake pedal stopped it for a while, then it came back. Ended with a longish break. Comes only when driving some 30 - 40mph, and only after a long trip.

Are my front brakes "on" a bit, and when getting warm this happens? Should I do something to it? What?

 

OMG!!!!! Change your pads! And also the rotors, since it would seem the steel backing to the pads has gouged into them rendering them useless.
Or, maybe not. Difficult to say without looking at them. Take it to a REPUTABLE shop who offers a Free Brake Inspection, and ask to SEE what they tell you is wrong.

 

Well, the seller said they have been inspected and were good...and his affair is the official Jag representative on this area. And the only one who repairs Jags. But I do not trust them...

 

Disc brake squeal/noise is usually from two sources: 1. Severely worn pads; and/or 2. noise from vibrating brake pads.
When pads are worn to metal you have metal to metal braking surfaces; and the noise will be all the time. Easy to check pad wear.
Vibrating pads are subtle. You wouldn't think they vibrate under pressure, but they can and that is ofter the source of intermittent brake noise. The cure/fix is to lubricate the back of the pads with special copper-based grease. If you are not a DIY owner, then consider replacing pads "while you are in there."

 

Considering these cars have electronic brake sensors that trigger a dash light I wouldn't suspect worn pads. As for vibration It is possible but I would be suspicious as almost all new pads have anti squeal pads on the back and don't require the brake grease . That being said it could also be the pad spring which could be causing the vibration. You would need to pull the wheel and do a visual inspection.

 

No dash light warning on my '07 XK when the rear pads ground down to metal. My "warning" was unusual brake noise.