Yes, you can use 28mm tires with Shimano Ultegra 6800 calipers (or virtually any other road calipers).
The brands you mentioned produce a multitude of different forks. I am fairly sure that most all of them would also support 28mm tires.
However, all of this also depends on a factor you did not discuss: your wheels. If your rims are too narrow, it will be very inconvenient to remove and install the wheel with tire inflated. If your rims are too wide, you run a slightly higher risk of interference with the fork. This is because a "28mm" tire is not really standard or as precise as the label implies: it could be 25mm to 30mm depending on the brand, the model, and the rim's interior width.
And there is another potential problem: you have chip seal roads there, do you not? The chips sometimes get stuck to tires, and when that happens, you will not be rewarded for having the least possible fork crown clearance.
In summary: you'll have to try it out. You might be forced to buy 25/26mm tires, or a different brand, if your chosen 28mm ones run on the wide side, or you have unusual rims, or you have debris where you ride. Consider buying one 28mm tire first to try out, and use it in the back if it doesn't work in front.