I doubt the BT 4K UHD is a 1080p upscale there would be uproar, and with the advanced HEVC encoding these days 4K with Dolby Vision streamed is possible on ADSL you only have to look at Netflix for that with an average bitrate of about 15mbps seems plenty enough for a 4K picture.
I have seen some of the BT feed uploads captured from PC cards and they are definitely 4K UHD (.TS file with Mediainfo ), i presume thats why when BT first brought out there 4K sports channel you had to have BT Infinity 2 up to 80mb to be able to stream from the BT vision box, and seeing as BT scrapped there upto 38mb fibre for upto 52mb fibre they guarantee that most customers can get the 4K Sports channel