In many parts of the country, parking 'offences' (dealt with by the police and/or traffic wardens) have been 'decriminalised' and are now just parking 'contraventions' (which are dealt with by council parking wardens). That might well explain why your local police force appear to be ignoring parking on double yellow lines. (i.e. it's no longer a criminal offence, per se, so it has nothing to do with them).
However, 'causing an obstruction' remains a criminal offence and the police can still act to deal with it. They can act whenever a parked vehicle obstructs the flow of traffic (irrespective of the presence, or otherwise, of any yellow lines) but, unless someone complains about the obstruction (or the parked vehicle is obviously causing a major problem) they'll probably leave things to the council parking staff.
Police officers need to think about which area they're patrolling. Here in Suffolk, the police are still responsible for parking offences in Stowmarket, where they employ designated 'traffic' PCSOs specifically to issue tickets to drivers who park on double yellow lines. However, if a Stowmarket-base police officer (or PCSO) is re-assigned to duties just down the road in Ipswich, he (or she) can no longer issue a ticket solely because a vehicle is parked on a double yellow line. (Parking irregularities have been decriminalised in Ipswich. Enforcement is down to council parking wardens). They can only act if the vehicle is causing an obstruction.
Chris