You cannot refuse to provide services to work-seekers or to provide them on the same terms as others because:
they’re not a member of a union or agree to become a member
they’re a member of a union or will not agree to stop being a member
they will not agree to make a payment (for example, to a union or charity) in lieu of union membership or to allow a prospective employer to deduct money from their pay to make a payment like this
Employment tribunals
Work-seekers can go to an employment tribunal if they think they’ve been unlawfully refused employment or the services of an employment agency on trade union membership grounds.
They can make a claim against either or both the prospective hirer and the employment agency.