Look further than just your Routers line clocked speed before deciding...
The router will tell you the clocked speed, BUT that is not actually the throughput you will have been allocated. It is often the case that you will actually be "squeezed" to half of that. BT has a system that messes with your line all the time, and both physical clock and vlan/pvc level. it is to try and make it stable at layer 1/2, however it forgets that IP can survive occasional loss.. (dear BT broadband designers it's called TCP)..but it is way to aggressive. UDP...well... depends on your apps needs.
In addition at peak time there is zero guarentee you will get anything over 2/4meg depending on your package, if you doubt this call customer service you will get a short and sharp answer on that one. If you have a good tool like wireshark you can see this in detail for yourself.
And...if you are on Option 1 you are gonna get dinged to 800k sinbin if you go over your limit.
http://www.speedtest.net/ works pretty well for checking out what is going on. Run it a few times over a few days to see your average.