Goik, The law is that a menber of a trade union is assumed to agree to pay a "Political Levy," in effect a donation to the Labour Party, unless he signs a declaration that he wishes to "opt out."
After signing such a declaration, it can take up to a year before it takes effect.

There is no necessity to donate the levy to charity.

As the member has to be a positive "Opt Out," there are a lot of members who do not support the Socialists who are still paying simply because they have either not been advised or simple inertia.

At least the situation is better than it was during the period of the now gone and very much unlamented Closed Shop when belonging to a Trade Union was often a condition of being able to get a job in the first place.