There are so many confused posts in this thread. I thought I'd come here and clear a few things up for you all - hopefully you can make more informed choices as a result.
Virgin Media are the UK's only widely available ISP who control every part of their network. They handle everything right down to the socket which you plug the modem into in your house. I'm paying them for 20Mbit/sec at the moment and I am getting very, very close to that.
Most of the UK's ISPs use a technology known as ADSL - it's getting on a bit, and it supports a maximum of 8Mbit/sec downstream - no more. The reality of this is that most of the ISPs simply re-sell connections which they get from BT Wholesale, meaning that at least at a local level, there won't be much (if any) difference between them all.
ISPs which do this are generally quite expensive - O2 for example will sell you 8Mbit/sec for £22.02 per month. There isn't much movement in the price either, as it's all set by BT and the competition is so fierce that your ISP's margins are probably extremely small.
If you don't live in a Virgin Media area, this is probably your only option. However, in some larger towns and cities ISPs are choosing to bypass the BT system in a process known as "Local Loop Unbundling". This involves the ISP coming to your local telephone exchange and installing their own equipment there, which can significantly reduce the cost to them of supplying your service and also affords them several technical freedoms.
The result for the customer is that the prices are usually lower or the speeds are far greater because they use a more modern technology known as ADSL2+. If you're really lucky it will be both cheaper and faster and this is where Be, O2 and Sky's good reputation comes from.
I cannot stress enough that the coverage of these LLU ISPs is EXTREMELY limited - in my home town we can get none of them whatsoever but YMMV. Visit
Sam Knows and enter your details and it will show you (on the right hand side) a list of all of the LLU ISPs available in your area. Check them out first.
Beware though, many of the LLU ISPs also offer a product to those who are outside of their coverage areas but as I outlined above, it is usually significantly more expensive and in my experience with O2 its performance is significantly worse because it's just something they're re-selling for BT.
Taking O2 as an example, if you're in their coverage area you can get broadband for £12.23 per month, but if you're outside of it then you have to pay £22.02 per month and will receive worse service, despite both being advertised as "up to 8Mbit/sec".
