In that situation the police would only give you a warning because the video camera in their car that would be used to convict you would also show them driving dangerously close behind you so they wouldn't be stupid enough to try and take it to court.
Im not advocating the endangerment of innocent people here im advocating trying to scare some sense into people already committing dangerous driving (following too closely)
Bull.
There is nothing in the highway code that says you must leave a gap of x. There is rule 126 which states that you should leave a gap but, crucially, not MUST leave a gap. That would mean that the only test left to prosecute the Police driver would be "the way he drives falls far below what would be expected of a competent and careful driver, and it would be obvious to a competant and careful driver that driving in that way would be dangerous". Under the circumstances and depending on the distances and speeds involved, a Police driver is probably unlikely to fail this test based on my experiences of where people have judged others to be tailgating but were in fact a reasonable distance back. Perhaps not the "two second" rule but as per my posts earlier in the thread, that is not demonstrative of an absolute distance.
Furthermore, save for a few special circumstances, committing one crime is not a bar to prosecuting someone else for another. If I assaulted you and then you went on to murder me, your murder charges would not be dropped because of the assault.
Brake-testing people IS dangerous driving and anyone that does it should wherever possible be prosecuted for it.