Been to countless London marts (sort of like a con but only dealers) over the last four decades and although I've seen plenty of L. Miller, Alan Class and Australian reprint books featuring American material from the 50s and 60s at such shows, I have never once seen a Streamline book at them, or indeed in collections, or offered on any British FB group etc. They never featured in U.K. price guides about American comics because they're British books (the irony!).
In fact I'd barely heard of them until you posted that. I know a couple of older collectors (one of whom specializes in British comics) who are in their seventies who were around when fandom began over here in the late 60s, and would know more about these rarities, but they've never mentioned their existence to me. Maybe Marwood and I would know something.
Either way, that is a cool piece.