TV devices are a hot mess to support from a streaming perspective, they each come with their own quirks that mean some perfectly-in-spec encoding and packaging techniques will result in a failed playback on some models of TV. Once a TV device _is_ supported, that support has to be maintained typically for more than a decade until usage of that model falls so low that dropping it from support can be justified.
It would be prohibitively costly to produce per-device renditions so instead there is one generic rendition produced for "all smart TVs" and another one for "UHD capable smart TVs".
Traditional TV manufacturers all work with the BBC to get their devices certified, which is a requirement for carrying the iPlayer app and comes with legal agreements that asset that a device _will_ be able to playback BBC content for as long as it's supported.
Because Apple like to Think Differently, they opted not to align with the entire rest of the TV industry in standardising on MPEG-DASH spec. They instead require all developers to stream video using the HLS protocol. As UHD content on iPlayer is geared exclusively for smart TVs, and all the other smart TVs support MPEG-DASH, the UHD workflow simply never evolved the ability to target Apple's TV devices.