(2) Any person who resists or wilfully obstructs a constable in the execution of his duty, or a person assisting a constable in the execution of his duty, shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding one month or to a fine not exceeding level 3 on the standard scale, or to both.
Police Act 1996 Section 89 - subsequent amendments don't seem to altered the prime wording.
(He was prosecuted for Obstruction rather than PCJ)
I still find that a little hard to swallow. He wasn't preventing the constable from carrying out his duty (which in that case was detecting motoring offences) in any way.
The most his light-flashing could have done is to prevent such offences taking place in the first place, so his role is one of crime prevention, which is surely to be applauded? 
Kevin
Welcome to the vagaries of the law.
If, by his intended actions, people were diverted from committing offences then I can see – in strict application of the relevant law, not in the most sensible application of it - where this offence applies here.
Mr Thompson seems to have been quite clear in his intention of alerting motorists - some possibly speeding (although how he could know for certain escapes me) - with a view to have those motorists reduce speed to avoid possible prosecution.
The altruistic element of his actions cannot apply in law as he appears to have had the intention of obstructing officers in this way by alerting others with a view to have them avoid being caught for speeding (if indeed any were).
Because of the way people are interpreting this matter it’s confirmed in my mind that this was an inappropriate prosecution better dealt with by way of caution and advice from the detecting officer.