You can only edit HTML within the tag of this email code, ie: the contents of and including the tag in your example code.

Ideally the Newsletter extension should be providing you with tools to edit the content, as this is specific to the email newsletter.

I presume the constraint of editing within the tag is imposed by JCE rather than some fundamental restriction elsewhere with Joomla.

It's not a restriction, but the fundamental way in which WYSIWYG editors in CMS systems work. Content is edited within the tag of the editor, so you cannot add another whole HTML document within this.

I wonder if you could explain the reason? The need to create responsive emails is overwhelming and as someone with limited expertise in the relevant CSS I am very much dependent on external support such as bought-in templates.

This is a very specific use case, within the context of a specific extension type, not general content editing for a website, for which JCE and most other WYSIWYG editors in CMS systems is designed.

It baffles me why these email / newsletter extensions don't deal with this themselves, by having additional editing options for adding data for the email structure outside of the email content (data within the tag of the email).

In the meantime I will reluctantly need to experiment with alternative editors to see if that will provide a workaround.

Your'e not going to find this within Joomla, as the default editor does'nt support this either. You could use the CodeMirror option to edit rthe HTML by hand.

I do have a plugin I am adapting that does allow editing of the content within the tag, but this will only be available in a few week.s