I'll add my frustration for this lacking (vital) feature to the above comments.
As a web dev, I was excited to start working with the Affinity products, but not having any way to see your optimised images BEFORE saving them renders the Expert Persona a pretty useless 'feature'.
You can't make an intelligent guess at the quality of the files you're working with and to be expected to move over to another piece of software to do a fundamental task like modify compression while previewing the changes, seems to be totally counter-intuitive IMHO.
Please add this feature in, so many of us are no longer reliant on the likes of Photoshop to work with web graphics.