Is it legal for me to prohibit my employer from calling an ambulance for me if I have a seizure? Because that would most likely be unnecessary and I wouldn't want to pay for it.
I think communication and mutual understanding is key here.
I don't know any place where you could legally forbid your employer from calling an ambulance. On the contrary, the employer is bound to care for your physical and mental wellbeing, a fact that is often ignored.
You should have a serious talk with your manager and the people working near you most of the time. If there are dedicated first aid workers in your company, invite them as well. Explain to them what a typical seizure looks like, how they should react to prevent injuries to you and themselves, and how long to wait until they call an ambulance. Be aware that telling them to never call an ambulance will most likely not work. It would help them immensly if you told them how often you usually get seizures and for how long.
For an untrained person, seing someone having a seizure is very shocking. The first instinct (and what every first aid training tells you) is to call the emergency number. If you're aware that a seizure is imminent, try saying "no ambulance" or something like that right before it starts.
You cannot block an employer from calling an ambulance for a medical emergency. I have diabetes, and even though a low blood sugar episode can be countered with orange juice, they will still call the rescue squad.
This is unfortunately, not your call, as it is an issue of liability. All that you can do is refuse any and all medical aid when they come.
Knowing whether it would be legal or not would be largely dependent on locale. That said, even if it were legal for you to provide some sort of waiver or agreement to them that said not to call an ambulance, as an employer I'd be deeply uncomfortable with such an arrangement, and I expect many others would be too.
You say that it would "most likely be unnecessary" - which implies that there is a chance it would be necessary. I'm not sure I'd like to be in a position to explain to anyone - lawyers, employer's liability insurers, and least of all to an employee's grieving relatives why we didn't call an ambulance when they were seizing.
Unless your employer and colleagues are trained medical professionals that can diagnose a seizure where medical attention is not necessary, there is nothing you can do to prevent them from calling an ambulance should you have any issue. That being said, they will not be charged for calling an ambulance and neither will you.
I am not a lawyer neither I am HR, bu if I were HR, I would probably frown upon this "prohibition clause" and instructs coworkers not to comply (and call an ambulance if needed), unless there is insurance that there is no risk for the company.
Why ? Imagine the following scenario :
You have a seizure.
Unfortunately, this is not one of your usual, or some nasty other thing happen in the same time
coworkers can't tell the difference, ambulance is not called.
You lose your life or left with heavy / irreparable damage
You or your relatives sue the company for non-assistance.
-> Company must pay heavy compensation (and may get some bad reputation)
(and, as an off-topic side effect, it's also bad for you too)
I can imagine such a "play with employee's life" clause possible if, for instance there is a already recognized medical staff that could take the responsibility to not call the ambulance in the emergency situation.
In any case, if you still want to take this risk for yourself, you should go see a lawyer and / or your doctor.
They are the people who can have knowledge of similar legal situations and find a way to make such a clause possible and riskless for the company, if it is possible.