OK lets put it this way, lets put it this way you have a length of cable that runs from your local exchange to your house. Now regardless of what router you place on that line you will only get what the line can provide given the distance between the exchange and you. Lets say you put a TalkTalk router on the line and it gives you a line speed of 5mbps (lets ignore line training for a moment). Now you put say a Draytek router on the lines and you might get 5.2mbps, this is not an improvement in the speed of the line but more a function of the routers ability to obtain and maintain a connection between the exchange and you.
When you fit a new router the router will give you the fastest line speed it can obtain. This is usually the most unreliable connection, over a period of up to 21 days the line goes through a period of training where it will start with the fastest line speed obtainable and then slow decrease the speed until the most reliable connection is made, this can be a couple of mbps slower than the first connection. This is why you should never turn your router off on a daily basis otherwise you will continually be going through this training period.
There are several different kinds on connection available from BT (basically all ADSL is supplied by BT as they own the exchanges and the cabling). Have a look at the link below, enter your phone number and click search and it will tell you a lot of information about the local exchange and what ADSL connections are available and when it is likely to be upgraded.
https://www.samknows.com/broadband/exchange_search