Even though I have a Fire TV stick 4K, have plenty of bandwidth, and have joined the iplayer beta for me it will not go past avc/1080p (I waited a few mins to see if it would adaptively jump up in quality)

The iplayer app on my Philips TV does show Dracula and" Seven Worlds" with a UHD badge

For me, with Fire TV stick display settings set at automatic, the stick upscales everything and always outputs over HDMI@2160p

I will try and report this to Amazon.