I wish to retract my claim that Stace's use of "native" was certainly
an error as I have been in contact with the man himself, querying his
use of it.

He replied saying that there is disagreement among botanists as to what
can be regarded as a native taxon! For hybrids, his own view is that
any that arose in the UK can be called native, whether or not its
parents are native (not least because quite often such hybrids occur
nowhere else). In the case of the Russell Lupin, although the hybrid
arose in cultivation, it also did in the wild, and it is this latter
occurrence that led him to use the term native in the New Flora,
reinforcing this with the words "spontaneous hybrids".

Makes sense. Thanks for following this up.
--
Kay