So with Grbl 1.1, all of the commands except the M6 line are accepted by Grbl natively. The G28 command should make it move up in the Z direction first before moving in the XY. It works on my install, so not sure why it isn't working correctly for you. The only thing I can think of is that UGS is somehow changing what is sent to Grbl. I don't use UGS, so can't help in that regard, but maybe you could try one of the other sending programs
https://github.com/gnea/grbl/wiki/Using-Grbl
Also, just a tip. It is always a good idea to do a dry run which can be accomplished by setting the Z=0 position to a location above the part such that the maximum z- move won't get down to the part, then run the job. The tool will move around above the part without actually cutting and you can watch its movements for anything that doesn't look right.