As part of the Pre-Action Protocol process, the claimant is required to send a Letter Before Claim to UKVI.  Your solicitor will help on this. The Letter Before Claim must clearly state why immigration decision is being challenged and the date of the decision.
Our lawyers have years of experience in drafting Letters Before Claim.  We understand the issue/s must be set out clearly, and the error of law must be identified.  Although the Letter Before Claim does not have to have the detail of a skeleton argument, it must enable UKVI to understand the basis of the challenge and how the illegality has arisen.  The letter must also explain what action UKVI is expected to take in order to remedy the error.