There are less prosecutions because there are less police to stop them.
// The number of traffic police in the UK has been cut by 12 per cent in the past five years, with some forces enduring cuts of up to 42 per cent, road safety charity Brake claims.
Numbers in Wales fell by 31 per cent and in England it dropped by 12 per cent. But traffic police in Scotland increased by four per cent, according to figures from a Freedom of Information request by Brake.
The largest cuts were in South Wales, where road police have been reduced by 42 per cent, and Dyfed Powys, which is down by 39 per cent. Hampshire suffered the biggest drop in England, with 36 per cent of officers cut. //
http://www.autoexpress.co.uk/car-news/consumer-news/65026/traffic-police-numbers-falling#ixzz2eogldKqh
There is a lot of evidence to suggest that the number using phones while driving has doubled since harsher penalties were introduced in 2007.