Sony has announced that the Android 6.0 update will start hitting Xperia Z4 Tablet and Xperia Z3+ units as well as Xperia Z5 series devices in the United Kingdom beginning March 7. The announcement was made by the UK subsidiary of the Japanese company on Twitter.
@FalseNugget it's set to roll out from March 7th for Xperia Z5 series, Z4 Tablet and Z3+ – we will share confirmed timings when its ready— Sony Xperia GB (@sonyxperiagb) February 15, 2016
What's worth mentioning here is that the tweet makes it clear that the date is not set in stone at this point in time. Also, given the fact that such updates from Sony have been off the mark in the past, at best we can say that the roll-out is 'likely' to begin at the said date.
For those who aren't already aware, the Android 6.0 Marshmallow update has already started rolling out to Sony Xperia Z5 (and possibly its Compact and Premium variants, too) in Turkey.