Hi wooky :)
regarding your troubles with proper DX, I wouldn't waste too much time with seeing 2 GP's they are not specialized with IBD, and personally I would continue going to new GI's until you find one that knows their stuff...
Here are 2 major differences between UC and CD that help docs determine which disease you're dealing with....
With UC the disease is limited to the colon and rectom with the entire area being affected with inflammation which only occurs on the surface of the intestinal lining...
With CD it can affect the entire GI tract, there are skipped patterns of inflammation (there's typically healthy tissues in between inflammed tissue) and with CD it can affect the many layers of the intestinal lining, ususally this leads to fistulas, something that typically never happens with UC because it remains on the surface only not penetrating beneith the surface as with CD. Also anal skin tags are related only to crohns because of the fact that crohns can affect the entire GI tract from the mouth to the anus, and with UC only the colon and rectom are involved.
If they're having troubles being able to tell from how the inflammation looks, as mild cases of CD could be confused with UC (no penetration beyond the surface, no fistulas and maybe just one small patch of inflammation that's noticiable) then the anal skin tags is pretty much a huge factor towards crohns and not UC.
Hope this helps :)
My bum is broken....there's a big crack down the middle of it! LOL :)