I finally got a UHD TV last week, already had Amazon Prime and there’s a fair bit of stuff on there (although the movie choice is basically none). The Grand Tour in UHD + HDR looks amazing though (in fact I’d say HDR is a much more obvious difference than HD>UHD).
Signed up to Netflix a couple of days ago and pleasantly surprised by the amount of UHD and Dolby Vision (HDR) content, inc. quite a few films.
Had Sky Q installed yesterday to, there seem to be quite a few movies in UHD now but not much of anything else.
Of the 3 I’d def recommend Netflix if you want UHD content (especially, as with Prime, you can do it on a month’s free trial and binge watch a shed load of stuff over Christmas…).