The general rule is that, "Candidates will be eligible for special consideration if they have been fully prepared and have covered the whole course but performance in the examination, or in the production of coursework or non-examination assessment, is materially affected by adverse circumstances beyond their control."
Seizures sound like "adverse circumstances" to me, and are presumably beyond your control. There are various examples in the
A guide to the special consideration process - General and Vocational qualifications, published by the Joint Council for Qualifications. (That link is dead right now, as they're having server issues, although the cached version is available as HTML.)
Your seizures might simply fall into the general category of "illness at the time of the examination", although it's possible they'd be classed as "recent illness of a more serious nature" or even "flare-up of a severe congenital/medical condition or a psychological condition". You will need to let you exam centre (school) know about the issue, such that they can make an application for special consideration on your behalf. Note that any all such applications "must be supported by signed evidence produced by a member of the senior leadership team."
If you actually experience a seizure during the exam, that would be sufficient evidence in itself, but to quote from one of the examples in the above-mentioned document, "As the centre knows that the candidate has been struggling with her health over the past few days, there is no need for them to ask the candidate to produce a letter from her GP."
Just to re-iterate: let you school know about the potential issue ahead of time, as it'll make it easier (for them) if there is an issue on the day. If there is an issue on the day, ensure your school applies for special consideration on your behalf.