You aren’t really losing anything in the 720 50p iplayer stream really on interlaced material.
The 720p50 stuff on iPlayer streaming is significantly lower quality than the 1080i25 on the Sky implementation though. The compression on the 720p50 streams is quite harsh, and the deinterlacing doesn't look too great either. (From memory 720p50 iPlayer is around 5Mbs and 1080i25 on Sky is around 8Mbs. I think Sky will carry 5.1 AC3 where available too.)
Compare something challenging like Strictly between the two and you'll see a clear difference.
Until recently I'd have agreed decent 720p50 and 1080i25 compared pretty well with each other, but the extra resolution is really noticeable in the 1080i streams when you are watching on a 2160p UHD display. 720p looks quite a bit crunchier...