This is the link to the PDSA.
http://www.pdsa.org.uk/eligibility.html
Even if you could have an MRI scan how is it going to help? OK you will know the cause of the fits, but to be honest it is unlikely that anything can be done about a brain tumour (especially at that age). And would you want to put the dog through all that stress just so you know what is wrong?
If you do have to make the final decision, be aware that you can call the vet to the house, or go into the surgery. You can leave the dog with the vet or you can stay and hold her. It is very much a personal choice, I have always stayed with my dogs, but I never condem anyone who cannot face it. I usually place the dog in a down position for the injection and they usually literally just go to sleep.
What you don't want is an experience like a friend had who had an old spaniel. The dog started to have a prolonged violent fit one night, she rushed it to the vet (a car journey 20 mins. long) the dog screamed all the way. At the vets they tried to bring it out of the fit for two hours. It took three people to hold the dog down. Three weeks later the dog had another fit. This time the owner just let it be and the following morning took it to be put to sleep. She said she wished she had done that the first time as she believed that it was unbelievably cruel to treat the dog the way it had been the first time. In the three weeks she had the dog back it was costing her 50 pounds a week in medication, the dog was nearly blind and deaf and was not enjoying his walks - really he was just existing.
Sometimes it is best to let go while you still have good memories and the dog has a good quality of life. Better too soon than too late. I have never regretted letting a dog go. Sadly it is part of owning a dog, but we owe it to them for all the years of love and