You don't need to enter the reservation number unless you want to take advant of the plus 10 but you will need to enter a phone number when you check out.
You can enter your uk number dropping the first 0.
When I had trouble retrieving our
DVC ressie I logged on to the DVC member site and checked what the primary number was. I then entered that number (Which had the 0 at the start) and it retreived no problem.
HTH