If there are enough people from the UK (or any other country) they could just have a UK journal. Then maybe at some point the US journal could be given to the UK people and vice versa. Also, the journal should be sized so that the shipping is as inexpensive as possible and people could just put it in their mailbox when they are done with it. That way it would be easier for people to keep the movement going. I think it's a really great idea! I LOVE getting mail!
I just ordered black and white sulky pens from uncommonthread.com They seem to have pretty good prices, but I didn't do a ton of shopping around to get the absolute best deal so you might find them cheaper.