Although it's not official, it's the number that has been generally accepted in the MoD, with reasonable assumptions on post 2015 budget... Whether a second or 3rd tranche of orders are made after 2020 isn't something I have any knowledge about, but certainly the planning at the moment is for 20/25 frontline aircraft. Does the order not have to be placed by 2014...get deliveries 2016???? and IOC 2018?...I don't know the exact timetable.
the ????? signifies that I think this delivery date is complete and utter bolleaux. Canada have just come out and said they don't expect to declare their jets operational until after 2020.