How do I publish a book in the iBook store for both the UK and the USA?
On the page you linked there is a paragraph about this.


  A U.S. Tax ID is required as part of the iTunes Connect sign-up and
  authentication process to sell books on the iBookstore. This
  requirement also applies to publishers outside the U.S.


You can apply here:
http://www.irs.gov/businesses/small/article/0,,id=102767,00.html

The online form is only available at these times:


  Monday – Friday 6:00 a.m. to 12:30 a.m. Eastern time
  
  Saturday 6:00 a.m. to 9:00 p.m. Eastern time
  
  Sunday 7:00 p.m. to 12:00 a.m. Eastern time