One slightly worrying/confusing reply from the Skyuser forum stated:
"It also depends on where Sky's kit is, in relation to your previous supplier's kit in the telephone exchange to where the trunk cables enter the building.
Another thing which can affect the connection speed is Fibre. Many members have commented on the drop in speed when it is installed in their exchange.
Since the cables supply the broadband to everyone, whether they are Sky customers or not, I would imagine that this is going to affect all ISPs."
That's obviously complete rubbish. The distance of the cables around the exchange won't matter as they're either copper carrying gigabit ethernet or ten-gigabit ethernet or most likely fibre connections with gigabit or ten-gigabit ethernet links.
There are two types of ISP in the UK, those that use BTwholesale's network to get the data from your home to their network - this includes PlusNet, BTretail (Infinity & Broadband), ADSL24, and many many others.
The others are the "LLU" providers such as O2/BE, TalkTalk and Sky. These ISPs install their own equipment in the exchange and crucially install their own network from this equipment back to their core network. (This link is known as back haul). They may rent this capacity from BT but at a dedicated speed - and the ISP will manage the use of this link - its not shared.
When an "LLU" ISP wants to offer Fibre, they have to upgrade their equipment at the exchange to accept the fibre feed from Openreach. This is known a GEA (generic ethernet access) and so far Sky and TalkTalk have done this - sadly O2/BE have not had the money to.
So Sky fibre users will be sharing capacity from the exchange back to the Sky core network with Sky ADSL users. TalkTalk fibre users will be sharing with TalkTalk ADSL users. BTwholesale based Fibre ISPs are sharing capacity with BTwholesale based ADSL ISPs. Its upto the backhaul network teams to keep the networks larger than the capacity being used.
We've seen all ISPs have "congestion" issues; and they are often in the news on the main site front page, but they are often resolved within a few months. (it takes 30/60/90 days to install more trunk capacity).
James BT Infinity 2 19/09/2012 - Estimate 44.6/6.5 - Install 52/12 - Actual 46 / 8 Mbps
13 years of broadband - ntl:(512k/1M)/BTbusiness(2M)/Metronet(2M)/Bulldog(8M/16M)/BE(16M)/BT FTTC(50M)