Thank you for the reply. It does answer some questions but also raises more.
This document is 6 years old and while there is mention of 2 datacentres (NA & EU) there have been a lot of changes in that time, largely that UK is no longer part of the EU. It also doesn't say which country/countries in the EU the datacentre is.
It also doesn't answer my main question which is whether we can choose where our instance can be provisioned, nor my question regarding encryption in transit or storage.
I'm afraid unless this information is readily available to the customer (which it is not) we won't be using FortiMail Cloud.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.