I assume you are referring to the "14-Day Ultimate" Ticket for UK residents? It's correct that you pay for 7 days and get 14, but because the UK tickets are specially priced, they expire 14 days from the first use and CANNOT be upgraded to No-Expiration. Those particular tickets will expire 14 days from the first use no matter what.
You can upgrade a ticket to No-Expiration anytime between the first use, and 14 days after the first use. (Basically anytime before the ticket expires, you can add days, No-Expiration, etc, even if the ticket has been fully used up in its original form. You will still be charged the full amount of the No-Expiration option. For example, if you bought a 5 day ticket, used all 5 days, then added 5 more days AND the no-expiration option, you would pay the no-expiration price based on a 10 day ticket, not just the 5 remaining days.)