If you are legally parked in a layby, then there is little he can do. The only way he can prevent you is by showing that the layby is closed to traffic or has been suspended - ask to see the traffic order stating this, not sure if this is law but they are usually displayed publically at, or near the layby.
Re: impersonating an officer if you are asked to 'move along' and have been given the above threat, then ask to see a. the traffic order and b. his warrant card, if the 'officer' refuses you have reasonable grounds to phone the police and report that you have seen a person impersonating a police officer. Once you give the location they may well confirm he is genuine, but nine times out of ten he'll get a call on the radio telling him to stop being such an officious prat.
It happened to me and the red-faced apology was great to see, Essex camera partnership now publicise the fact that they use police officers not civvies.

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Gordon Brown saying I got the country into it's current economic mess so I'll get us out of it is the same as Bomber Harris nipping over to Dresden and offering to repair a few windows.
Chaos, panic and disorder - my work here is done.
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