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It could be that your line has been connected to the wrong ADSL port in the exchange or the ADSL port has been configured with the wrong customer details. That is the only way your modem would default to that username. You will have to call support to have this issue fixed.
For ADSL an upload speed of 0.6 Mbps is not uncommon. The speed depends on distance from exchange. The maximum upload speed is 1 Mbps