That speedtest result looks around what I'd expect, and doesn't seem slow on an ADSL2+ service. Your line is connecting to the exchange at 19147. That's this part of those stats:
Bandwidth (Up/Down) [kbps/kbps]: 1,279 / 19,147
A sync speed of 19147 will give you a connection profile from the line management of 16887. This matches our systems. The throughput speed (the speed you see on a speedtest) will always be slightly below this so a throughput of 15.7Mb/s is what I'd expect really.
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