And a very interesting first posting, too, Richard. Welcome aboard.
I checked two sites for the density of Scots pine, the wood these people use, as against Douglas Fir. One said slightly heavier the other said slightly lighter, so I think we can assume that the are much the same. Since I started building my boat, I have come to believe that we don't actually need superb timber for our masts. Moreover, this wood is slow grown so is probably a better option than a lot of fast-grown Douglas fir.
If I were making a wooden mast, I should want to put on a layer of glass and epoxy. This certainly wouldn't be possible over creosote, but I'm not sure that the copper preservative would be compatible either. However, I'm sure Calders and Grandidge would know and, if they were incompatible, might be persuaded to sell an untreated pole.
Arne has some really good advice on making masts from a whole tree. I'm afraid I can't link to it - it's just too difficult on a tablet, but think it's under Junk Information/Public Domain Files ...