Some companies have regulatory requirements or customer contracts which mean they must store their data ("data residency") within the UK.
In late 2016, Microsoft opened datacentres in the UK (Cardiff, Durham and London), making it possible for companies who need UK data residency to have it. Previously, the nearest available Microsoft datacentre was in Dublin, in the Republic of Ireland, although UK customers could also select other Microsoft Datacentres including The Netherlands and the United States as possible locations for their data to be stored.
The Republic of Ireland is of course within the EU, but that may still not satisfy those who need UK data residency, and Brexit is unlikely to improve anyone's sense of security in that regard.
Where is my data?
If you sign up for most Office 365 services today in the UK, your data will automatically be stored in the UK datacentres. Which one(s) is not open for choice right now, but you can see this by opening your Office 365 portal as a Global Administrator and going to:
Settings -> Organisation Profile -> Data Location
and you should see something like this:
What if I signed up a while ago?
Well, this article from Microsoft says they would have notified you "when there is the option to move data", and they would have done that well before September 15th 2017 when the offer to do so expired for existing customers.
So far, we've not been notified about the option to move our data, and nor have any of our existing Office 365 customers.
Watch this space, we'll update this article as more information is available on this.