The term manual workers has changed in its meaning over the years. In that, over the first eight decades of the last century the term manual worker was used to describe someone who worked in mining, the steel industry, the road haulage industry or any occupation that requires heavy manual handling. Many of those occupations no longer exist due to globalization or are now largely mechanised due to the introduction of the Health & Safety at work act 1974.
In modern times, many of Britain's so called manual workers are not in reality manual workers at all. They are people who are employed in call centres, the retail trade, home delivery drivers, the NHS etc. They in their millions are Theresa Mays "just about managing" which are those that have joined Labour recent times or see in that parties polices meaning for them.
Outside of that group are the professions, many of whom are drawn to Labour, especially in the greater London area (what have become known jokingly as the chattering classes).
However, in many former Labour strongholds, many who have traditionally supported Labour have in recent years supported UKIP due to immigration bringing large-scale ethnic change to many areas very quickly. Unfortunately for Labour it was in the years of the Blair/Brown era it was witnessed as that immigration and ethnic change being at its highest.
Therefore, those who wish to see a return to that centre left socialism in the Labour party should think on that it was those policies along with the banking crisis and the Iraqi war that indirectly have brought Jeremy Corbyn and much further left polices to the fore