Thank you, Rudi, Gugie and Neal. I figured about 319 mm or so, but good to know it may be larger than that. I had fleetingly made a mental note to measure/calculate the BB height. This raises another question, I guess: what's the "practical" diameter difference between, say, a 27" rim with a 1" or 1-1/4" tire, and a 650B rim with a 42 mm tire? A larger tire at lower pressure will, seems to me, have more "drop" when loaded; but using a narrower 650B tire is going to have a smaller diameter than a wider tire on the same rim. So is there a good way to figure out how much lower the BB will be than it was on the stock bike? Maybe I'm overthinking, and just need to build a 650B wheelset, and try it out.
One other factoid: this is a frame on the larger side (the way I like them) with 62 cm/24.5" CTT seat tube -- would that have an effect on the BB height, compared to a 57 or smaller? I'm statistically in the latter range stature-wise, but somehow feel more comfortable on the taller frames. And of course, I can't change that about the Raleigh.
If I were using metal fenders, I'd certainly consider bending them, so as not to lose or interrupt the edge roll; with plastic, I think that coping them works fine.