Aside from the distance concerns - which is why you use Cat cable and hang either HDMI Baluns or HDBaseT Baluns off the ends instead of long HDMI cables - the other two technical issues are HDCP and EDID.
HDCP you already know about. It's there to protect the film or program from being copied or displayed outside of a secure digital chain.
EDID is a digital handshake to establish hardware compatibility. This is where most ad-hoc/DIY systems fall down.
Mixing TVs and distribution gear and source devices is bound to involve different specs. For example, all your TVs could be 4K, but maybe only a couple of prime sets are ATMOS compatible, so now, without something to manage all the EDID signatures, your Sky box is seeing both ATMOS and non-ATMOS hardware in the system but can't serve both and so resorts to the lowest common denominator. You can also get ripples in the distribution system when TVs turn on or when the source selection changes to the HDMI for Sky. EDID can be a bit of a pain.
There are manufacturers who specialise in high bandwidth HDMI signal switching and distribution who have these EDID niggles nailed. The rub is the cost. It can run in to the thousands for something with almost guaranteed glitch-free performance.
Whilst you're picking yourself up off the floor, I would ask why you need to distribute 4K at all?
I'll preface the following by saying that I am a Sky customer too, and I think they've got the best PVR ecosystem around. However, IMO, their 4K offering is letting the side down.
There's very little real UHD 4K material broadcast live by Sky. Most of it is catch-up/download. What you're watching live is, for the most part, the same HD 1080i and SD 576i content that you've been watching from a standard Sky+HD box. The fact that a Q box can be set to output at 3840x2160p (UHD resolution) simply means that the box is upscaling 576i and 1080i to a UHD resolution. Your 4K TVs do that anyway, so why spend a huge chunk extra s=to pipe upscaled 1080i round the house?
That's not the biggest disappointment though; it's the lack of HDR and WCG.
If push came to shove, I'll bet you can't reliably tell the difference between upscaled 1080p and true 2160p based on resolution alone for the typical screen size-vs-viewing distance we use in our homes. The film company Paramount did blind testing to the same effect. Even those with better-than 20/20 vision couldn't manage it. It turns out that resolution alone isn't that important above 1080p. Unless you have a home projector with a true UHD imaging chip-set, or you sit incredibly close to a larger-than-average TV, then there's very little benefit.
What does make a huge difference is the bigger contrast range of High Dynamic Range, and less-so but also noticeable the bigger colour range of Wide Colour Gamut.
Compare UHD alone versus UHD with HDR and WCG; the difference is immediately visible regardless of screen size and viewing distance. Sadly though, Sky's UHD content doesn't yet support HDR or WCG; not even when watching Netflix which does have the full-fat-content versions via its direct service. The bottleneck for Sky is their 2TB Q box. It doesn't do HDR and WCG.
You might be better served keeping the Q box signal for the main TV and distributing the signal from a couple of mini boxes instead. At least that way it would be possible to watch some different Sky channels and box sets/films.
