To clarify how your line is made up - you have a dedicated pair of wires from your house to the exchange, possibly several kilometres worth.
Along the way back to the exchange, more and more pairs of wires will be in the same cable, typically up to 100 pairs of wires as it enteres the green cabinet at the side of the road, then in 200 or 400 pair cables back to exchange. But you still have your own exclusive pair of wires all the way back to the exchange.
This pair determines your sync rate and line stability.
At the exchange, your line splits, via microfilters (in effect), voice goes to the exchange linecards, data to the DSLAM. The DSLAM has a number of Backhauls going off via the BT ATM network back to POPs, then from these to your ISP via Centrals (normally 622Mbps), then through the ISP's network onto the internet via their transit links.
If you get a good, fast, stable sync, but poor throughput in general, this is generally down to either exchange congestion on the backhauls (which would affect multiple (but not necessarily all) ISPs), the ISPs Centrals (most likely), or the ISP's own network and onward transit links.
Very simplistic view, but should be enough info
Almost simple enough for me but not quite, sorry J, but step by step,
if the line is ours and ours alone to the exchange then,
if sky use bt exchange and line then no improvement as said
if o2 use their own eqipment/exchange on the same site,by passing the bt exchange(as im lead to believe) but on the same dedicated line then we get an improvement?
hope so, as fast running out of other options. 
LLU providers use their own data (or voice or both) equipment in the exchange, but use the same pair of wires to your house.
This presents advantages - as ISP owns the DSLAM, they can provide, for example, ADSL2, and there own BRAS profiles, and they have their own dedicated links back to ISP (representing significant cost savings for ISP).
This can, and sometimes does, result in better throughput, but not if the problem lies with the capabilities of the copper pair to your home. True, they can tweak BRAS profile (compromise between stability and speed), and ADSL2 can increase (and decrease) a given line's sync speed, but ultimately the differences will be minimal if its the line thats limiting. "You can't put a shine on a turd".
Solution - get the line fixed, if faulty (remember, the 2 operators, BT and Kingston Telecom, only have to provide a voice service). Or move.