I used to have Sky same time as BT Youview and a side by side comparison showed to 3 different family members each indicated the BT HD picture looked clearer and sharper. Same TV.
Not very scientific but I could definitely tell a difference. However whether the figures match this I dont know.
I'd look into registering your interest for FTTC since you do need that as said already.
So far as picture quality goes, as an ex Sky customer I found little difference. By far the biggest difference is caused by the variety of quality of broadcast material I think, such as on Discovery etc. IPTV can be better optimized like Netflix because it does not need real time muliplexing. There is certainly no reason why it can't technically outbeat sattellite, no Sky channels use any thing like the 50Mbs available I'm pretty sure.
Old Sky HD boxes should out put 1080 I'm sure that was the only real reason I could see a difference when I got the BT box, it was set to 1080i which I believe means the TV was doing the de-interlaceing. I have some tie thought the picture is a bit jerky on some shows but not sure if Sky was like this too.
So far a teresstrial FreeSat vs. FreeView I preffer freesat, however some channels are missing from Freesat, noticably 4HD has regrettably recently dissapeared for some reason. You could keep your old Sky HD for FreeSat but I think they still charge £10 a month for the privillege so best to invest in a new Humax box if you want FreeSat too,