Dear Alastair Leith,
Good day!!
The reason behind Anyone with the link can view is grayed because the SharePoint Online Admin or IT admin of your Office 365 organization has disabled external sharing in the SharePoint Online Admin Center. If you want to share the Excel file in the cloud location with the Anyone link, please check with your IT admin whether external sharing can be enabled on the site where the files are stored.
If the IT admin can't enable the external sharing due to company policies, the alternative can be downloading the file from the cloud location to your device and upload it to your personal OneDrive to share it with Anyone link. Note: please make sure whether its company policy.
Thanks for your patience and understanding.
Have a good day!!
Best Regards,
Sophia