It is all rather murky, as it depends upon the attitude of the individual Police Officer.
I know someone who brought an ex Police motorcycle at auction & about a month later was stopped & charged with impersonating a Police officer as it was an ex Police bike, with a white fairing that still had orange reflective tape on it.
Fortunately in that case a fellow motor cycle club member was a solicitor & when the Police realised that their definition was going to be challenged,they suddenly dropped the case. Something they would not do when the victim of their stupidity was the one challenging it. When it was just a motor cyclist, it was we will let the courts decide. When it was a solicitor & a claim of a malicious prosecution, it was "having reviewed...."
Local to me a company who provides private fire cover at events was charged with using a vehicle fitted with blue lights, (that were not covered,) as he went shopping. Yet the Police, fire & ambulance service do it. Also people such as St John Ambulance use their vehicles on non emergency contracts, runs etc with the blue lights uncovered. I have also moved emergency vehicles around that were fitted with blue lights uncovered & never had an issue.
As I understand the law it is "BLUE LIGHTS" that are the issue, not reflectors. But even if I am right, it does not mean some idiot Police officer would not try to get his/her own way, or that they would not be backed all the way to court, unless you spend money on a solicitor.
I think it was here, where someone said they reported a burglary & when the Police arrived they were only interested in the ex police radio they owned. These radios are sold at auction & perfectly legal to own & even use, if you have a licence & use it legally. But the idiot Police officer saw an easier case to solve & confiscated the radio, something he had no good cause to do, as the confiscate it, he needed to have good reason to suspect it would be used illegally. Which he did not even seek to find out.
What is or is not legal & how the Police respond are at times two very different things.