Actually, those two maps probably provide a very good proof that a large proportion of the population are entrenched voters. Between 1910 and 1970, most of the working men in those areas would have been miners, and most would have voted Labour religiously. Their wives would very likely have voted Labour too. Their children would have grown up learning that Labour were the best party for their family, and most of their friends would have had Labour-voting parents too.
And those sorts of ingrained voting preferences can last way beyond the closure of the mines - as these maps seem to illustrate very well. There are still legions of people in those areas voting Labour because their parents and grandparents were staunch Labour supporters, and because voting Labour is still seen as "the right thing to do" in those areas.