<Nerd mode on>I assume Lightning (II) was thought to be a good thing politically for US-UK relations.
If it was to give a nod to the UK EE Lightning it would have been the
Lightning III, as it is the
II in recognition of the P-38 Lightning from WWII.
Similarly, of course, if the Panther was anything more than an unofficial nickname, it would be the
Panther II, in recognition of the Grumman F9F Panther of the early 1950s...