By the way, note that the Bluefly Polo boots also appear to have the script "Ralph Lauren" on the insole.
This doesn't mean they are EG, they put that script on some C&J made shoes as well. I think those boots are likely C&J.
I wear an 8D and wear a 7F in Church's. Width fitting aside, Is the EG 89 last a fairly true medium width?
Yes, but it is important to understand that English D and English E widths are both really medium widths. D is slightly on the narrow side and E is slightly on the wide side. However not narrow or wide enough to correspond to a US C or E. A Church's 7F are probably equivalent to a US 7 1/2 E, not an 8D. If your toes aren't cramped, then it sounds like you wear a US 8 for the sake of width, not length. These Cardiffs (likely a 7 1/2 / 8 E) will probably fit nicely. If anything they will be a little bit loose - and you should be able to take care of that with a bit of padding under the tongue or a thin insole.