Just to stick my oar in, even if a bit late - I saw another example of why undertaking could be dangerous the other day: As one car was undertaking a van in the middle lane, another cars was simultaneously overtaking it - they then both planned to move into the middle lane in front of the van, luckily they both saw each other in time and avoided an accident - but I could defiantly see this happening if the law was changed.
I know I'm echoing what a lot of people have already said - but what we need is education and enforcement of the current law, not a new one.