TLDR: Very slow cashback (7-8 months).
I went for regular Sky Broadband with the evening and weekend calls package. Speeds are around 18meg down, 1meg up. Although it was stuck on 8meg down for about a month whilst they claimed nothing was wrong and finally it turned out to be a faulty chipset at the exchange. Lots of complaining and phone calls later and it's running decently now, never drops out.
The modem/router is very basic, can't issue WOL commands, doesn't support the DDNS service I use, poor WiFi performance etc. My main reason for sticking with them is the speeds are very stable, don't drop much during peak times and they don't do any bandwidth throttling or shaping (at least as far as I can determine).
Still haven't received any cashback to date, but after 7-8 months it is now confirmed and due to arrive this week. Was meant to be £50 upfront and £50 after 6 months IIRC, so that's out of the window.