Nicola,
the eye screening is not an eye test, but they will get you to read from a chart briefly. It is a better form of the photo/ophthalmoscope examination that your optician does at a check-up.
They will photograph your retinas without the interference of your iris and examine the photo for retinopathy. That photo will then be used in subsequent years to look for differences.
As others have said, get your BG under control, then go and see the optician - unless, of course, your eyesight has deteriorated to the extent you need new glasses for driving.