Where I grew up in the mid 1980s, within a 20 mile radius or so there were steel works, and closer still coal mines and sources of employment involving manual/unskilled labour. The kids who hailed from those areas and backgrounds (the boys anyway) I think were hoping or expecting work that didn't require too much thinking. There was also a lot of peer pressure amongst them not to do anything considered high brow such as like reading novels for pleasure, listen to classical music or enjoy paintings. It was considered 'cool to be thick'. Ironically enough not long into the 1990s de-industrialisation had set in with coal mines and steel works closing like they were going out of fashion (which I suppose they were in a sense), leaving people who'd underperformed at school up the creak without a paddle.
For non-Brits 'thick' in this context means stupid or dumb in the intellectual and academic sense. 'Thick as two short planks' also 'Thick as sh*t'.