What would we (UK) do with the source code anyway?
after not very much time you'd end up with (from a software POV) 2 completely differant aircraft
American JSF
and
British JSF
the 2 differant source codes would be incomptable, so if the yanks produce an update for AMRAAM it wouldn't work on British JSF's (or their customers) without the brits producing their own update
and if the brits integrate ASRAAM it wouldn't work on US JSF (or their customers) without producing their own update (likley?)
repeat ad infinitum
their really can only be one code to rule them all
by asking for 1xmoon (on a stick) and settling for less we (the UK) have either been very clever or very stupid and only time will tell