Great link, though it doesn't mention anything about 'using a mobile phone whilst driving'. It does mention speeding or dangerous driving, which is what should be reported. I think the obsession with mobile phone use is wrong. According to official figures, it accounts for maybe up to 20 deaths per year, and this figure seems to have increased since it became a criminal offence.
Bottom line is dangerous driving is dangerous driving, IF it's caused by mobile phone use, report the dangerous driving. If it's caused by smoking, report it, radio us, kids, whatever the cause, if it's dangerous driving, report it. Don't report somebody for dangerous driving it they are not, whether you personally approve of mobile phone use behind the wheel or not (and I should make the point that I don't), but to report somebody for dangerous driving purely on the basis that they are using a mobile phone, I would say is a false report.
According to official statistics, mobile phone distraction is a factor in 0.8% of accidents, nearly all are simply caused by driver error. 3.2% are caused by bad road layout. For full details see link below.
http://www.telegraph.co.uk/motoring/road-safety/8702111/How-do-accidents-happen.html
It's up to the Police to enforce the law, the link is useful to assist them in their duty, but it should be used properly.