Right, well, the maximum altitude you can
legally fly here in the U.K. is 400ft from the point of take off, so you were getting close to three times this altitude!
My advise is set the max altitude at 400ft in the GO app.
1,500ft is the maximum (vertical)
altitude attainable, which is set in the software and this cannot be overridden. The
horizontal distance for the P3 within Europe is 2.1 miles, albeit I have achieved 3.2 miles outside the U.K. using a windsurfer; local conditions do have a significant impact on performance however, such as wifi interference, trees, buildings etc.
However, the maximum
legal distance you can fly in the U.K. is 500m; this is to maintain VLOS (Visual Line Of Sight) so in other words, so you can still see it!