Sony won't release the sim-free update until the major physical operators approve it. When I had a Sony Xperia Z1 this meant roughly 5-6 WEEKS after everyone else. Cause of problem? T-Mobile.
Note that there's another update been announced; so now it's:
17.1.A.2.55 -> 17.1.1.A.0.402 -> 17.1.2.A.0.189
Don't blame Giff Gaff for this; it's the Voda's / O2's / Orange's / T-Mobile's of this world. And of course, Sony and their stupid release process.