The habitat is not always reliable (though Iād be willing to put large populations in āancientā woodland down as Silene dioica) - Iāve seen the two species growing side by side. (And the hybrid is quite common.)
I thought it interesting that the AI suggestion for the post you cite is is S x hampeana.
For many years the propensity of S. dioica and S. latifolia (S alba as was) to hybridise has been recorded; more recently molecular studies have given further insights which are not necessarily, (but not necessarily not) helpful in this case.
In the field the two species seem to be ākept apartā by ecological (as in the Poland & Clement key) rather than genetic systems and wherever they meet hybrids occur. These are intermediate in characters between the parents. The hybrids are fertile and they backcross so it seems reasonable to expect to come across plants across the whole spectrum. I am wondering if the hybrid and the generations of backcrossing are what we often see in lowland Britain.
I have just noticed that the front cover of Stace 2019 is S. dioica var zetlandica from Shetland.