Unlike cable broadband connections ADSL broadband is a technology that is distance sensitive. As you know from using a landline sometimes you hear cracking and noises on the line, the same noise affects ADSL broadband as well and in effect slows down the connection. Voice connections use wide array of voice amplifiers, loading coils to improve the call quality, these improvements are not compatible with ADSL connections though.
There are devices that improve broadband signal quality and reduce noise - these are called micro-filters and many consumers have them fitted in their homes. The most common one is BT iPlate which has been developed by BT and has been proven by many consumers that it can speed up your ADSL connection.
Essentially, the standard ADSL can work up to 5.5km from the exchange. However, to get 8Mbit you would need to be located no further than 2km from the exchange.
To understand why noise exist we need to look on how traditional telephone works. Historically, copper wires were used to transfer voice using the frequency range of up to 3400 Hertz. However, the frequency potential of these wires is much higher and led to a discovery of ADSL technology(Asymmetric Digital Subscriber Line). ASDL uses available frequencies on the copper wire (that are not used by voice) and divides them in assumption that users will want to download more than upload - that's why it is called assymetric.
Any signal that is carried through the wires is affected by characteristics of Resistance, Capacitance and Inductance. These characteristics directly contribute to more noise on the line and their effect is greater on a longer lines.