Russell presents some of the differences in social class in Blood Brothers as being by-products of the enormous gulf in economic opportunity between the working and middle classes.
In fact, the themes of class differences and the gap in economic opportunities between the classes are inextricably linked in the musical. Russell wrote Blood Brothers at a time of mass unemployment in Britain, where many of the old industries such as shipbuilding and coal mining were in the process of serious decline.
At the same time, certain sections of the population, especially the aspirant middle classes, were enjoying greater prosperity due to the free-market policies pursued by the conservative government under Margaret Thatcher.
As a consequence of these policies, however, the gap between rich and poor widened, and Russell explores how a simple change in environment can make all the difference between someone having a good, prosperous life or a life marred by poverty and lack of opportunity.
Had Mrs. Johnstone not been forced by economic circumstances to give Edward away when he was a baby, there's every chance that he would've turned out like his brother Mickey, a man who's had a hard life as a working-class man in Thatcher's Britain.
Russell's conclusion appears to be that, in a fundamentally unjust country, which he believes Britain under Thatcher to have become, environment is the most important determinant of what kind of life people lead. That explains why Edward and Mickey, despite being brothers, lead such completely different lives.