If you want to join a trade union. You can find information at http://www.tuc.org.uk . Joining a trade union, usually after a qualifying period. Gives  access to advice on employment matters. Where you & your employer disagree. They can be useful in advising you or provide a representative to attend meetings with your employer.

Whereas most employers will have processes to deal with employee problems. Usually referred to as a "Grievance Procedure". This being the first mechanism used for any employee to raise an issue. Should matters progress. The trade union may be able to provide more work focused legal support.

They will certainly be able to advise you in relation to seeking "reasonable adjustments" etc.

You should note. If you have an existing issue then join a union in an effort to then get help. They will usually not deal with any issues prior to you joining and as aforementioned you may be subject to a qualifying period. Usually a few months. After joining before you can access services.