well, its how much speed can be pushed over "x" distance.. Since your in a house which you don't know what BT connection would actually get, it may be that your are getting your best.. and your quite far from your exchange..
When your margin drops to about 5 you line will be unstable and you will suffer from packet loss and possible disconnects.. So sky will try to give you the best speed, while keeping your margin stable above 6... or they should..
However, the low margin and connection speed could be down to bad wiring in the house, an unfiltered socket, the line from the telephone pole outside your house is old and or damaged. Many things...
When your on the telephone, calling someone, is the line clear, no crackling or noise.. ? if not then the line from the telephone pole is probably fine.
Also, on the master socket, it is possible to have a filter there, so the router has a dedicated line, direct from the master socket. This removes the need for filters on each socket in the house and the noise generated by the rest of the house.
If you do both of them, then you are probably getting the best you can from your ISP... unless your modem is faulty and connecting slower, and has increased margin, happens very rarely but i have seen it happen.