Where is the Conservatives' Zell Miller to say it?
As an absolutely bare minimum in each of the 11 mainland regions, you need at least one Commons seat, at least one European seat (not at all a difficult thing to obtain), and either or both of the Leadership and the position of the single largest party on at least one principal authority.
If you are without any one or more of those, then, by definition, you are simply not a national political party.
At present, therefore, there is only one national political party.
Astoundingly, that is not either of the parties to the governing Coalition.