What is bread rusk ?(MaybeNewSausageMaker!)

Today I was a bit flabergasted when my daughter presented me with a
small poly bag of Hog Casings.
( It seems that a while ago I had mumbled something about trying to
make our own sausages but didnt know where to get the skins ! )

So, I googled a bit and came across lots of refs to including bread
rusk ( and some refs to yeastless rusk as well)

What is Bread Rusk ? will I find it in my local Tesco tomorrow, or
where ?
I dont suppose it is a fancy name for bread crumbs , is it ?
We have lots of those because we make our own bread.

That ref to yeastless rusk suggests that the other is infact some sort
of leavened-cooked-flour mixture or am I way off ,,,

hope that makes sense, if not blame it on the couple of gl***** of
cider wot I needed to recover from the shock of the gift !