Surely, in a modern economy, a modern Trade Union should be fighting for its members' best interests, which are not necessarily as above.
Absolutely. Which is why the now combined group is ultimately better than what came before.
Could be something to do with the fact that some TUC affiliated unions are led by people who, instead of merely representing their members, also have their own personal political agenda.
That's the nature of the beast. Can't run a country / union / political party / business / organisation / club without some element of politics.
It obscures the ideal/best solution for the greater good. But you have to hope right will out. What other choice is there? Communism? We as voters / members vote or choose those who represent us. We must vote them out/ in when we have the chance.
Ignoring political agendas or avoiding them will not make them go away.