Hi @Rupert Cook, welcome to the forums!
If I am correctly understanding what you are asking for, you can achieve something similar in Affinity Designer by taking advantage of the Export persona. You would create a single page large enough for the entire design and create the design as if meant for a single large page, then in the Export persona would create slices the sizes of the individual pages and configure them to export in the desired format (PDF for example). After that you can export them all at once using the "Export Slices" button; a separate PDF will be exported for each slice.
