Your connection is probably capped (I have a friend with Sky who's connection is capped both down/up). You will need to ring Sky to get the cap raised to removed.
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Downstream Upstream
Connection Speed 8128 kbps 448 kbps
Line Attenuation 13.0 db 10.0 db
Noise Margin 16.8 db 24.0 db
Max(Kbps): 11616 1056