White pudding, or mealy pudding as it's sometimes called, is an oatmeal based food, made with oatmeal (primarily), spices, and ground sausage. In consistency, it's similar to black pudding (or blood pudding), without the blood. You can check it out on the internet.
"Pudding" is sort of a misnomer, as pudding is now a British term for dessert. Some menus in Britain don't have a "dessert" list, but a "pudding" list. If a waiter or waitress asks you if you'd like pudding, it's not necessarily custard or Jell-o!
You'll find white pudding and black pudding on the menus at many chippies in Scotland, along with haggis. Unfortunately, they are breaded and deep fried, which does nothing for the taste. Best to try it somewhere that doesn't deep fry. A B&B, perhaps.
Hope that clarifies white pudding for you.
p.s.: I enjoy poutine, as well!