My TV broke six months ago, so replaced with a 4K one. So then got Sky Q to get 4K stuff to watch. There isn't much 4K stuff on, and for my new TV screen size and viewing distance, I can't really tell the difference between 4K and HD. I knew in advance this would probably be the case. I figured most content would be HD, so the best TV size was the biggest that was good for HD. For the record, my view distance is just over 7 feet, and screen size is 55 inches. To get the benefit of 4K pixels, ideally I would have to go to 65 inches.
For 99.9% of households, their viewing-distance to screen-size ratio is going to mean that the difference in number of pixels between HD and 4K is of no benefit.
Having said that, more pixels is only one of the differences between HD and UHD, others are twice as many frames per second, which might be beneficial when watching sport, more different colours, and a wider colour range. (To explain the difference between the last two, more different colours means for example extra shades of red between existing ones, whereas wider colour range means shades of red outside the HD range.) Another difference is that the dynamic range, difference between brightest and darkest parts of the picture, is bigger. (Sky don't support this yet but probably will in future.)
In short, the eventual benefit I hope to see from UHD is better pixels, rather than more pixels.
In order to get get UHD from Sky you have to have the larger Sky Q Silver box, and pay £12 a month multiscreen subscription, even if you never watch TV on anything other than your main screen.
The Sky Q silver box has 12 tuners, which can get all get their signal from one input cable. So if like me you live in a flat and have only one satellite feed, Sky Q means you can record more than one channel at the same time for the first time. This was a major benefit for me, I can schedule as many recording as I live without ever worrying there won't be a spare tuner available.
Of the 12 tuners, 5 or 6 can be used for making simultaneous recordings, two can be used to supply live TV to miniboxes, two to supply live TV to tables or phone, one is for watching live TV, one is spare. (Think there might be possibility of having one channel in a window while watching a different one, which takes another tuner, but I haven't come across how to do that, so may be mistaken.)