I cannot say why employers are not so keen on English from lower tier universities but it might be what is studied and maybe they think it’s a little self indulgent?
Sorry been out of range for a few days – and Happy Christmas everyone!
I might have some insight into this @TizerorFizz as DH is a writer and a younger member of the family recently completed an English degree.
In my humble opinion (well, DH's to be honest), the traditional universities, including redbrick as well as RG, tend to make it hard to avoid studying a broad range of Literature through the ages and also you are likely to have to study novels, poems and plays – it would be virtually impossible to avoid one whole genre or even one whole century. Obviously you can angle your degree to favour a certain era or genre but you will have to study some stuff you don't much like!
The former polys (and again, this is a generalisation) are more likely to offer 'themed' modules, for example Class and Culture in Literature, Society in Literature, the child in Literature, or Literature and Colonization. Nothing wrong with such an approach per se, but you can see immediately how much easier it is to focus on a narrow range of works/authors who you already enjoy. There is nothing there which will take you out of your comfort zone and study something you might prefer to avoid.
FWIW the latter graduate may be no less employable than the former, but they will undoubtedly be less well read, generally. An English degree which doesn't oblige you to study a single one of Shakespeare, Chaucer, Milton, Austen, Dickens, Joyce, Hardy, Bronte (either), Conrad, Lawrence, James, Wordsworth, Eliot, the metaphysical poets, the war poets, Pinter or Stoppard leaves a lot of gaps…