I think this is just journalists looking for a story.
They sell over 2 million cars a year in the UK - stock needs to be parked somewhere.
It is true that during the recession stockpiles of all goods, not just cars built up but since 2014 UK had had a record car sales so any excess stock will be long gone.
They stretching things by implying the cars got buried like some video games. A lot of consumer goods get destroyed/incinerated/deep buried rather than be sold cheap on the market but I'd be surprised if they did the same with cars - they have a long shelf life so no harm holding some stock and writing it down..
The reporters claims of cars rotting and falling apart by being parked are funny, maybe for US made cars (oops that's Z3's then) but any car will survive just fine being parked up for a year or two (maybe not the ones in Sheernesss, they probably all got nicked!).
Quite amazing that Sheerness made it onto international news actually!