In the early days of adsl bt used to us contention ratios, but the did away with this years ago.
Bandwidth is allocated per virtual paths (VPs). On some of the old Dslams the VPs could be quite small. On 20cn exchanges all maxDSL users are usually put on an MSAN which has superVPs. A maxdsl premium aCcount will be on a different vp to normal maxdsl as these are allocated slightly more bandwidth. Even so contention and congestion can still be an issue if bt get caught out and don't upgrade the VPs quick enough.
For adsl2+ and 21cn exchanges, they still use VPs but congestion is much rarer due to the structure of the backhaul. There's info on the main site about this.
No preferences are given for sync speeds, just natural congestion and preferences for some protocols. If by any chance congestion kicks in, then the higher rate lines will notice first.
For example if the vp is busy, then a 24Mb line would notice first and this could slow the line down to say 12mbps. A line on the same VP syncing at 16mbps would also be slowed to 12mbps Yet a line syncing at 10mbps or less, wouldn't even be aware there was an issue.
Think of it this way, I'd you have a busy motorway and heavy traffic slows down speeds to 60 mph, then all cars are slowed down to the same speed regardless of what speed the car is capable of doing. The Sunday driver on the inside lane doing 50 mph may think its busy if he looks out of his window, but he's still free to tootle along at 50 and isn't affected.
With vdsl its natural congestion again but there's a couple of places local congestion could occur. 1. Dslam to exchange and would affect all users on that dslam. 2. On or near the OLT which could affect all vdsl users at the exchange due to insufficient capacity on the backhaul for the vdsl VP (btw based fttc) or SP backhaul capacity in the case of TT/sky.
Gea fibre providers such as sky and TT also have a 3rd possible location and that's on the GEA fibre link between the OLT and the ISP backhaul. This is the link between BTs equipment and their own.. So they must also ensure that they purchase a big enough link, otherwise it will slow down all their fttc users on that exchange.
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Damnit I'd typed a long post but the ipad lost it
. It sometimes does this with long posts as it attempts to do a full page refresh. I've tried to type most of it again, but because it's taken so long and I don't want to risk another preview... I've not proof read so apologies for typos etc