I cancelled Sky on the same day I got it. It was shockingly bad. The guy on the phone told me the Sky broadband gets throttled at peak hours especially.
Not worth it for an extra 10/20% that means you internet is useable. Although we dont have fibre optic.
Not true, sky do not throttle and downloads are unlimited.
Your first mistake was going by the first day speed as you put, they adjust the speed for the first couple of weeks to see what you're able to achieve, so it will start low and increase all the time.
I moved from flawless o2 broadband to free sky unlimited and get the same speed of 21mb download, so far I can't fault it. Never been throttled.
When we're talking LLU broadband services, then there is contention though, so it will affect your service depending on how overloaded the LLU equipment is at the exchange you're connecting to. I've never really experienced this, but the number of people who use bt means you can get this on bt broadband ( not fibre services I don't think).
As as for the line owner prioritising their customers, I think that's bullshit, the line is the telephone line, it's the LLU equipment at the exchange doing all the work and the that's owned by the company you pay.
If it's not LLU then you are sharing the bt service and in that instance it might happen, but I don't think there's much of that about these days, most providers have LLU equipment in most exchanges now.