There are so many things that can affect download speed. How far are you from your exchange, the quality of your line, the router, your isp, etc. Many of your questions are covered in this thread broadband speed
Check your exchange here and see if any ISPs have their own kit in the exchange (known as LLU). they will be able to offer the best speeds.
For reference I live in a pretty rural area, BT could offer me a shade under 2mb/s download, so I went with Talk Talk, when I checked at the weekend I was enjoying 10.43mb/s download and .88mb/s upload
By way of interest I saw the broadband speed test and then saw the offer of a 'free scan'. If you click on this you get Uniblue Registery checker - or something like this - which you can install on your machine. When you run this the programme will go through your machine and tell you that you have all sorts of problems which it can fix!! Just one teensy, weensy problem - you have to pay to get this!! so much for a 'free' scan!! Please beware of this.