Could be either. Also might be in Australia or Singapore, and more regions are planned. See https://www.atlassian.com/trust/reliability/infrastructure
By default, cloud will use the location nearest to your main activity, so if most of your users are in London, it'll probably land in the Republic of Ireland, if France or Germany, Frankfurt, if they're antipodeans, Australia, and so-on
Premium Cloud users can select and "fix" the region their data is held in.
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