As an ex resident of Kent I have some knowledge of the restriction. It was well publicised when it was introduced, there are signs in lay-bys and on affected roads etc. The problem is that truckers using the ferries and tunnel use any available space to park. Residential streets, industrial estates, lay-bys such that it can be difficult even get around.
They generally ignore double yellow lines in quieter areas where they wont get nabbed, double park, obstruct lines of site near entrances etc and mostly because they dont want to pay at a truck stop. They also leave rubbish, excrement and bottles of yellow liquid dotted around.
I know they have to have somewhere to park but fairs fair.