Although I don't see any realistic argument as to why changing the age to 18 will help reduce the accident rate amongst young drivers, I do think that there is a problem with the system as it stands now, when 21.7% of UK road deaths in 2011 involved drivers aged 17-24, who make up just 7.2% of drivers.
I think the change should be made to the learning process. For example, they could introduce a minimum period of learning of, say, 10 months from the first lesson, and a minimum number of lesson hours with a qualified instructor (say 30 or 40 hours), like is the law in a lot of other European countries. This could include compulsory lessons in motorway driving, night-time driving and urban/rural driving. They could also have a minimum period on the road in accompanied practice like in Australia, recorded using a log-book, and a compulsory first aid course to get a full licence, like in Germany. That way, new drivers would be less likely to encounter totally unfamiliar situations and better able to deal with them.