You are protected against being fired or refused a job because of trade union membership or activities, including activities in the past. Your employer is not allowed to treat you any differently if you are a member of a trade union. This means that they must not pass you over for promotion or training opportunities, or treat you differently from non-union members in any way. The same right also applies in the unlikely event of discrimination in favour of union members.
In addition, you have the right to be accompanied by a union official in a disciplinary hearing. You can also have someone accompany you in a grievance hearing if the grievance relates to your terms and conditions of employment.
If these rights are denied, or you are sacked for anything related to trying to exercise your rights, you have a case that can be taken to an Employment Tribunal; though it should be mentioned that they are stacked in favour of the employers with little compensation available.
For more information on your rights at work: www.stuffyourboss.com