Yes, the rims are the same diameter, although the sort of rim that's good for a 23mm tyre won't be so hot for a 2.3in one.
You'll have trouble getting fat MTB clinchers to sit on a narrower rim.
You say that, but my MTB rims are only 17mm wide and seem fine with 26x2.1" tyres. Also, when I say 700c tyres, I don't want to use super skinny 700x23c race tyres, maybe 700x35 or 700x42.
You are comparing a UK handmade frame with mass-produced Taiwanese.
I fully appreciate that and as much as I love cycling, would I be able to tell the difference? After all, we're not talking about a custom, made to measure on-off, so to some extent we are talking about where it's made, not how it's made, because as far as I know, even in Taiwan, frames are hand made, I don't know of any manufacturers that are robot welding/brazing frames. Although I also appreciate that we're comparing time consuming hand fillet brazed with tig welding. If only I were rich and money were no object.