I could be wrong but, I'd take a guess that Sky would ensure that their 4k content could never be recorded on an external unit. (Not in 4k anyway)
Even with HD, broadcasters were pretty choosy with what you were allowed to do. I played with a Panasonic Blu-Ray recorder (with Freesat tuners) for a while - you could record HD to its internal HDD. The machine had the ability to burn HD content from its HDD to its Blu-Ray disc recorder - but only if the broadcaster allowed it. At most they allowed only one HD disc to be burned, sometimes none at all.
If anyone makes a UHD HDD/Disc recorder in the future, it's likely to be Panasonic, maybe with satellite tuners, but, even then, I'd expect the same, or more, copy restrictions - certainly no native 4k out to external recorders.
Keep looking though, people always find unofficial ways of doing things don't they?