Ethel:
Unmarked police cars around here in Suffolk definitely have blue lights behind the front grill. I've worked on traffic surveys where uniformed officers have stopped traffic, so that I can interview them. When unmarked police cars approach the survey point, they always switch their 'hidden' blue lights on, to identify themselves to the uniformed officers, so that they don't get stopped.
Super-Vixen:
Mouthing a 4-letter word (of the offensive type) at someone, other than when in a dwelling, is an offence under Section 5 of the Public Order Act 1986. The maximum penalty is a fine of �1000.
However, if the action is done "with intent to cause a person harassment, alarm or distress" it then becomes an offence under Section 4A of that Act. The maximum penalty then rise to a fine of �5000.
In practice the police normally deal with such matters through an �80 fixed penalty notice.
Chris