I think you will have to phone Sky. You may be one of the few whose throughput is effected by an exchange fault. It does not happen often but when it does it is difficult to get fixed as it requires your 'card' to be changed in the exchange.

Phone Sky and emphasise that you are calling about your throughput, not your sync speed, and would like to be put through to the ones who handle BT escalations. You might have a difficult time as tier one are bad enough with PC problems like this, they will be lost with a Mac.