There is nothing specifically requiring a pre-action letter to be sent by solicitors.
However, it is fairly technical and ideally requires knowledge and experience of Judicial Review and all the law involved. The danger of a ‘do it yourself’ approach is that you may make an error or leave out something important, which will simply mean that you have wasted time and haven’t saved any money because either your solicitors (when they get involved) have to redo the letter or, if you start proceedings, the case gets thrown out of court.