Hi there OhLaLa Fresh branches with 1 to 3 years growth are what they need, while there is still juice in them. It was just a suggestion in case you have some you can prune easily. For the concentrates, sheep need particular gut flora to be able to digest grains and other stuff in them and it takes them a while to build the flora up. For this reason it is best to give them a small amount of concentrates every day through the winter against the time such as now when they will have to eat larger quantities as nothing else is available. I think many people have been caught out by this sudden severe weather, with hoggs which don't know what this new food is.
The hard frosts have now effectively killed the grass. It needs day and night temps above 7 degrees C for grass to start growing again so they will need hay plus small quantities of concentrates from now until you can see fresh grass growth in your area.
Because your snow is so deep they won't be able to scrape down to grass. Ours manage until it's a foot or so deep but after that they just rely on hay and whatever else we give them, plus whatever they can stretch up to in the hedges. Our black sheep currently all have white faces with the snow which has stuck to them from ferreting around below the surface.
Something else we give our sheep - the older ones - is digestive biscuits. Many people will throw up their hands in horror at this, but sometimes they will stimulate the appetite when nothing else will. Your hoggs won't know what they are though so would ignore them too. A way round that is to push small pieces into their mouths, behind their front teeth, so they get the taste. After that they will come for them. Not large amounts, just tidbits to perk them up.
Do you have any brassicas in your garden? The outer leaves of those usually go down a treat too, and taking them doesn't stop your crop.