What is the "Exchange"?
Some light reading. http://www.kitz.co.uk/adsl/telephone_exchange.htm
It's the BT building in your town (or possibly a nearby town) where all the electronics for telephones and ADSL broadband lives. It's the connection point between your telephone line and the rest of the BT network (both for phone and broadband). It's where the DSLAMs live ( http://en.wikipedia.org/wiki/Digital_subscriber_line_access_multiplexer ).