If a law firm asks direct questions on their application form regarding candidate’s medical records you should be honest and disclose that you have had/have epilepsy.
Firms arent allowed to discriminate against medical circumstance and epilepsy doesnt usually stop people from working. Firms application forms often provide an additional information section, which I would advise you to use and explain in a factual way how your grades were affected by epilepsy which will give the firms an explanation as to why your grades were poor.
You should end in a positive way saying how you have overcome your perceived problem and properly managed and controlled your condition. Also if youre on the LPC you could include on your statement something about your progress on the course so far and what you expect your final result to be.
Remember this is a small part of the application form and you mention that you believe you meet the core requirements that firms are looking for so ensure you play on all your strengths throughout the rest of the application.