I think you need first to establish why the bill has increased so much.
The problem you have is "he said, she said". You've no proof that the vet quoted £1K as opposed to estimated it. They could argue that they said it will be "about" or "around" £1.5K (although I accept that the difference is huge).
It's all too late now but I'd have questioned the need for the MRI in the first place because it's unlikely to make a difference to the limited range of treatments available IME. I'm not a vet, I rescue cats. I've had a cat who fitted due to previous brain damage, in her case caused by abuse.
An MRI is, if I understand it correctly, to rule out other causes of seizures rather than to diagnose epilepsy. Epilepsy can be ideopathic (no known cause) or caused by, for example, head trauma following an accident or abuse.
The standard practice in my experience is to first wait to establish if the fit is a one off or whether they continue and/or increase in intensity and frequency. The need then would be to stabilise before the cat continued to fit to the point of going into status (not coming fully out of one fit before going into another). This is extremely dangerous, life threatening and can lead to further brain damage.
Then the decision usually (ime, sorry to keep emphasising this!) would be to medicate with a drug - phenobarbital is commonly used, either alone or with bromide, also known as Kbr) although there are a couple of others. One, gabapentin, is in use more in the US than over here, it depends on how clued up on epilepsy your vet is as well as the cat's needs which will determine the medication.
So, I'd want to know why there was such a big difference in price and what the vet planned to achieve with the MRI. I'd think you might be on a hiding to nothing but if they can't give you solid reasons for suggesting an MRI (accepting that whatever is done, epilepsy, if that's what it is, can't be cured, only controlled) I'd argue that the price increase plus no good reason for the investigation are good grounds for at least a goodwill gesture of a reduction. Your problem is that your vet didn't do the MRI, another did, and your vet will be passing their bill onto you. It would be interesting to find out whether your own vet has added a mark up to the actual cost of the other vet's scan just for the being the middle-man. If they have then yes, I'd be asking for that to be knocked off.
As an aside, if your cat needs daily medication it will be less expensive getting a prescription from your vet and buying it online than it will be buying it directly from the surgery. Dietary changes can help as well as reducing stress and triggering stimuli. Keep a diary of fits, "partials" (where the cat seems "out of it, not really with it" and maybe staring into the distance or repeating an action unnaturally) to try to eliminate the offset of a fit.
I hope you don't need that advice and that the fits of the weekend are never repeated.
Finally, it's of no help to you but I plead with owners to insure their pets. These sort of bills are horrendous but provided that there's no previous history the average policy as low as £8 to £12 per month will cover them.