Post Confluence 8.3 release, XML space export doesn't have the 'Custom export' option which is present in PDF and HTML exports. 'Custom export' option for XML space export is beneficial when a set of pages needs to be exported instead of the entire space.

Below are few workarounds that could be helpful.

Workaround 1

• Access <baseurl>/spaces/exportspacexml.action?key=<spacekey>
  • Replace Base Url and space key in the above link
• It will land on the page which contains Custom export option for XML 
• Go ahead and create custom XML export for the desired set of pages

Workaround 2

• Create a temporary space and copy relevant pages into the newly created space
• Once the required set of pages are present in the temporary space, take an XML export