Hello Simon WDP,
Thank you for posting in Microsoft Community! We are happy to assist.
I understand your frustration and I do apologize for the inconvenience. Typically, in the Classic Outlook for Windows version, the File tab is located in the top left corner of the screen. However, if you are not seeing it, it seems you are using the New Outlook Style and that makes it a bit confusing.
Thus, based on your description, you want to be able to control the email font properties in Outlook. Feel free to correct me if I misunderstood.
Therefor, to change the plain text email compose type to HTML, you may follow these steps to get to the point where you can switch to HTML
First Change the Ribbon to Classic View --> Click View > Layout> Classic View

Second, Change from Text to HTML: Create a new Email -->Click Message in the right of the Ribbon, click Switch To HTML
If your scenario is different from the above, please kindly provide us with the screenshot of the Outlook interface you are using and your Outlook version details? See What version of Outlook do I have? - Microsoft Support
I look forward to your update. We are happy to provide any further assistance if needed. So do not hesitate to reach out!
Best regards,
Microsoft community