It will have been noted on your optician's medical records. If it's not changed for years, then they're unlikely to say anything.
Nystagmus in itself doesn't preclude you from driving:
This is the standard which you need to achieve to be able to drive.
You must be able to read (with glasses or contact lenses, if necessary) a car number plate made after 1 September 2001 from 20 metres.
You must also meet the minimum eyesight standard for driving by having a visual acuity of at least decimal 0.5 (6/12) measured on the Snellen scale (with glasses or contact lenses, if necessary) using both eyes together or, if you have sight in one eye only, in that eye.
You must also have an adequate field of vision - your optician can tell you about this and do a test.
So, your visual acuity will have been written down on your records from the optometrist. The visual field test you need to do is called an Esterman Visual Field, which is done with both eyes open.
If your visual acuity is lower than driving standard and it has been stable and documented over many years, I cannot see how they could insist on an ophthalmology report. Optometrist's records are medical records.
So, first port of call would be to speak to your optician and see what your visual acuity is. If it's less than 6/12, then ask for a letter and copy of your records. I'd explain what it's for as well.
If it's better than 6/12, then you probably do need to see an ophthalmologist for a report. Nystagmus is quite complex and can cause other symptoms in people which would make driving difficult, such as being very sensitive to light or having to turn their head to a certain position to reduce the level of nystagmus - which in turn would reduce visual field.
HTH