They must be "forloughed" i.e. still employed by their firm, but not working for it.
I'm not at all sure that was made clear.
gov.uk/government/pub…
But if a firm needs all 10 employees to operate its machinery (for example) it can't use the scheme to cover 80% of the wages of some/all of them.
If it's open to all, some/many firms will abuse it to save on their wage bills.
On the other hand...
If a firm has seen a 75% drop in revenue because of the coronavirus, it's very possible it can't keep trading with 25% of staff.
So there's a big gap.