Hi Keith,
Thanks for your Private Message.
As I understand it, you have added the .com domain and verified it in Office 365. Now you want to redirect .co.uk address to .com address. Please correct me on time if I misunderstand the issue.
I have checked the website address, the .com address and www.domain.com work fine. Meanwhile, I noticed that the Name Server points to Office 365, if so, Office 365 only supports one public-facing website, so the redirect (from .co.uk to .com) is not available in Office 365 if you point the Name Server to Office 365. Thanks for your understanding.
However, to achieve your requirement, as a workaround, please remove the Office 365 Name Server records from your domain registrar, and then add the Office 365 service required DNS records to your domain registrar manually. After that, you may rewrite the URL, such as using URL forwarding to achieve the URL redirect (from .co.uk to .com).
Please refer to the link below to add or modify custom DNS records for your domain. The required DNS records are listed under the DNS manager tab.
Thanks,
Eric Sun