The road traffic Act 1988 says you must stop for a constable in uniform when requested to do so.
Sussex police offer the following advice
The police stopping procedures are as follows:
If the police want to stop your vehicle they will, where possible, attract your attention by: • flashing blue lights or headlights or sounding their siren or horn • directing you to pull over to the side by pointing and/or using the left indicator.
It is exceptionally rare for police officers to be impersonated. However, if you genuinely feel concerned or in doubt as to the authenticity of a police vehicle signaling for you to stop, drive slowly to the nearest lit built up area with members of the public nearby or go to the nearest police station.
Remember - Unmarked police cars will stop you with blue, not red lights.
Sounds like a modern legend to me. Lots of these things circulate around .It only takes one employee from Dorset Police to forward it from a Police computer and it looks like it is endorsed. Any UK unmarked Police vehicle should, ironically, have very clear systems for indicating that it is a Police Vehicle e.g. pop up lights saying Police, Blue strobe lights in the grill and the occupants should be in uniform to stop you. I would certainly agree that anyone should not stop if they are uncertain if it is a genuine Police car until such time as it is safe to stop. Common sense should prevail with the officers ( Hopeful I know) as to where they stop vehicles.