Over the horizon
The key to understanding this decision (or any government decision, come to that) is that by the time this carrier comes into service and it's shiny new aircraft arrive - either horizontally or vertically - there will have been an election. No government has the ability or will to look further ahead than the next ballot, as they'll either be out of power or have new and more interesting problems to
screw up solve. By that time, or even further ahead when/if an enemy emerges that needs the might of an aircraft carrier's planes to defeat it, nobody will remember who decided what (and those who did decide will all be on the boards of various defence companies, anyway) and how to bring them to book.
I expect this decision was not driven by strategic thinking, but by expediency: JFDI or CYA or both. As it is, the chances of a government official being able to outwit a company that's dedicated its existence to squeezing as much money as possible out of it is slight. Even if such a brainiac politician was in the right place at the right time, the defence suppliers only have to wait until the next election for that person to be reshuffled and replaced by someone more "amenable".