I wonder if anyone can answer me this; would say a 50mm or 85mm prime focus at the same distance from the subject as my zoom would at the same focal length? Or does that change with lens construction? Sorry if that's a daft question. I'm thinking of getting a nifty fifty to lighten the load a bit.
It depends on the particular lens concerned. Rather than looking at the closest distance at which you can gain focus, I think it is easier (because you can look it up easier, and it is probably more relevant for what you are thinking about anyway) to think in terms of magnifications.
The Canon 18-55 EF-S has a maximum magnification of 0.21X.
The Canon EF 50mm 1.8 STM also has a maximum magnification of 0.21X
The Canon EF 50mm 1.2L has a maximum magnification of 0.15X
The Canon EF 85mm 1.8 has a maximum magnification of 0.13X
The Canon EF 85mm 1.2L II has a maximum magnification of 0.11X
(You can look this stuff up in
the Lenses section at dpreview, for example.)
So, in terms of magnification with the bare lens, looking at these particular lenses, the 18-55 is as good as it gets for magnification. Image quality, of course, is another matter.
I imagine the relationships between the maximum magnifications of these lenses are the same when you add extension tubes. (That's a guess.)