As I said, 3Ts, this initiative does NOT involve any new laws. All it has done is to provide the police with a simpler way of dealing with the existing careless driving law. This is similar to the fixed penalty measures which were intoduced for speeding in (I think) the late 1980s. Prior to that all speeding offences had to be dealt with in the Magistrates' Court.
Careless driving, by its nature, is subjective. All the law (Road Traffic Act 1988, s3) says is this:
"A person is to be regarded as driving without due care and attention if (and only if) the way he drives falls below what would be expected of a competent and careful driver."
And that's all, nothing more specific. It is up to the police (initially) to decide whether, in their opinion, the driver's standard has fallen below the required level and then the court (if necessary) to hear evidence and reach a verdict. What this new initiative has done is to allow the police to offer, and the driver to accept, a fixed penalty for careless driving and in particular for the examples under discussion here. In more serious cases of careless driving they would still issue court proceedings.
Many drivers have been prosecuted (through the courts) for careless driving involving tailgating and incorrect lane discipline.