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Actually, if you click View -> Studio -> Glyph Browser, you can place the cursor where you want the glyph to appear (if inserting) or highlight the area you where want the glyph to appear (if replacing text) and then double click the 1/2 glyph (it is on row 27 from the top in my view).

Actually, it is possible to take a single image (or even several) and export multiple files at different sizes (but as far as I can tell they must be of constrained proportions). Get your image prepared however you want in Affinity Photo. Click on Export Persona. Using the slice tool, Select the whole of the image. In the slices pane, make sure the slice you made of the image is selected. Click what looks like a video play icon on the slice in question (it opens up a menu). In the field that displays '1x' put your first desired width dimension (make sure to add a 'w' or a 'h' after the numerical value so it knows which dimension to use as base). In the other field, create a renaming system (i used [Slice name]_[width] for mine). Click the '+' to add more of these widths(for each set the renaming pattern). You can click on the Export All icon, and tell it where to save the export files. Make sure to save this and name it so you can reuse it as often as you need it. The above steps leave out some of the nitty gritty details of saving the 'Macro', but it didn't take me long to figure it, and it won't take you long either. Hope that helps someone.

+1 on this - it is a basic feature of modern web development and it is actually difficult to believe that this functionality doesn't exist already.