I would have thought that this situation is quite unusual.
*In our Wills, if a DC pre-deceases us, their portion will be left to their 'issue', which I understood was a normal thing!"
Did the 'Deed of Variation' not include your cousins? I realise that for all sorts of reasons, your DF might have considered that inappropriate, but it does seem sad that they lost their Dad and also the inheritance from their DGM.
My dad hated his brother (to be fair, with good reason!) and, by extension, my cousins. Though in that family uncle was the golden child.
I've not seen the actual deed of variation, I only know for sure that me and my sister got money. No way would dad have left out my brother (he's the golden child, but he and I don't speak) so he will have done too. Dad told me it was done by DOV.
Years later, I asked mum if dad gave any money to the cousins and she said he gave them a small amount. But no idea if that was included in the deed or not. I do need to see it really, but not til mum dies, but I might ask her where it is.
I don't think it was that unusual all those years ago not to include issue, as my parents wills were both the same (they all, grandparents and parents, said "to my issue and if any have pre deceased to the remaining issue" or similar, specifically excluding the next line down, who may not have existed when the will was written of course).
By the time uncle died, grandad had already died (everything went to nan) and Nan was already too far gone to be able to rewrite the will anyway.
My cousins won't have any idea the size of the estate though, they will assume it was all spent on care as Nan was in care for over fifteen years. But it wasn't because my dad managed the money very carefully and she got most of her care paid for by the NHS as it was nursing care. Unless they look up the probate record which I'd say was unlikely really.
Obviously there is all sorts of other history around it, like one of my cousins not being my uncle's kid, as their mum had an affair, which we all knew but she didn't and found out when she was in her fifties via a DNA ancestry check thing, then other cousin told me and I said we all knew (I didn't know they'd never known!).