There is a legal requirement for a vote to be held for the establishment/continuation of a Political Fund in a trade union, it is held every ten years. The establishment of a political fund enables trade unions to campaign in the political arena - not the necessarily the party political arena - on matters that impact upon their members.
Without this positive vote trade unions are unable to campaign on matters deemed as 'political', even though they impact directly and indirectly on collective bargaining or individual workers rights. Examples of matters that are deemed as political have included subjects such as privatisation, health and safety, employment legislation and even pensions.
A political fund or political campaigning by trades unions does not mean affiliation by a trade union to the Labour party. Indeed many of the trades unions who took part in last weeks Pensions strike are not affiliated, including FDA, ATL, and Head Teachers association! And anyway it is not compulsory for individuals to pay the political levy, and it is very easy to opt out.
This change was introduced in the 1980s by the Thatcher Government in the name of individual freedom and accountability, yer right, more about weakening the trades unions! If it was about individual freedom and accountability then why did they, or indeed subsequent government, not introduce legislation to demand that companies setup a political fund, and a regular individual shareholder vote for when they campaign in the political arena, or indeed make large political donations to political parties, usually the Tory Party!!