Right you are... the contract was written that way to avoid the syndrome of partners proposing to buy large numbers of aircraft, thereby inflating their share of engineering and manufacturing work, and then scaling back.
But the pain of it is that now - with CVF and JSF committed and locked together - the Govt now announces that it can't afford JSF and Typhoon, and the UK gets JSF, which is slower, shorter-legged and less versatile.