The guys I find usually working at the key cutting type places aren't really the type to get their knickers in a twist over a toy gun. I got a small Spyderco knife done a while back, no issues with that at all. They are usually the sort of hard grafting blokes with paint-stained clothes, rough accents and a lot of experience with tools and old-fashioned metal/wood/leather work in general.
It's going to vary of course, but as long as you take the gun apart as far as you can and only hand over the minimum parts necessary, I doubt it'd be any problem. I remember some years ago I had the gearbox out of a SCAR-L that I needed to take apart but the screws were all torx heads that I didn't have the tools for at the time. Took the gun fully apart and brought just the gearbox down to B&Q inside a bag; nobody saw the thing, but they'd never have known what it was even if they did. Similar thing with a lot of parts, if you fully stripped an AR upper for example, 99.9% of the British public would not know what that random cylinder of metal actually was.