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Hi Foxview,
The “active since 2016” is the active time for the new data center geo. It’s not related to the active time for Office in-app chat feature. Refer to the reference article, the United Kingdom is on that list, that means the Office in-app chat feature hasn’t been supported for this region yet.
Really sorry for the inconvenience, but more and more regions will be enabled this feature. We appreciate your understanding and patience for that.
As the workaround, you can choose to use comment function instead of the chat feature. Or you can choose to vote this UserVoice: Make the Chat feature built-in Office application available for all locationsand keep monitoring this updates.
Best Regards,
Rena