The effects of a high Kp index will be more pronounced the nearer you are to Earth's Equator and to the poles. That means that, as indicated above, for latitudes going through the likes of the U.S.A. and England, say, the effects won't be so significant and UAS flights in those latitudes would still be possible where a high Kp could be problematic if taking off within the Arctic Circle, say.
For the moderate latitudes, then, one should still be aware and take notice of the Kp index and if it is elevated then take extra care and certainly keep the aircraft in VLOS in case you do lose your satellites.
Through my own choice, since I do tend to be on the cautious side, I generally won't take off with a Kp of more than 4 but I will consider taking off with a Kp=5 if all other conditions are favorable. Definitely no more than 5, though.