If a decision is taken by the DVLA to revoke or refuse a licence, there is no internal appeals procedure. You can ask them to reconsider their decision and if there is further medical evidence available this should be sent with the request to reconsider. The evidence will be considered by a medical panel.
The DVLA should have given its reasons for the decision to refuse or revoke a licence in writing. They will also give notice of the right of appeal to a magistrate's court as per Section 100 of the Road Traffic Act 1988. However if you declared a disability which requires mandatory action under the regulations there is no right of appeal.
An appeal (or "complaint") to a magistrates court must be made within six months. You must establish your case "on the balance of probabilities" which means that you must satisfy the magistrates that on balance you are more likely to be right than the DVLA. This will almost certainly require evidence from a medical expert which may be difficult to fund as legal aid is not available. A local citizens' advice bureau may be able to offer some free advice and assistance.
The courts have little experience of these sorts of applications. However, the making of the application itself requires the DVLA to review its decision to be sure that they can uphold it, which can in some cases result in the return of the licence before any hearing takes place. The DVLA will attend any court hearing and they are likely to be represented by a lawyer. If you lose your appeal then the DVLA will probably ask for an order that you pay any legal costs they have incurred.