NTL's limit has basically nothing to do with what their network can sustain (although there ARE some limitations at their internet gateway -- which all the local connections have to pass through).

What it boils down to is they think it's an easy way for them to reduce costs AND increase profits.

If you had to pay an additional 0.1 pounds per every extra GB/day you downloaded (in a 14-day period) -- then they could make more money off the 'bandwidth hogs' so long as NTL could support that kind of useage.