I'd re-iterate what Niknak said. Have your prescription handy.
I did this prior to going for my Class 1 and they were keen to give you the best possible outcome. I passed the opthalmic and made another appointment to do the rest.
I originally failed the CAA Class 3 medical over refractive error but now have a class 1.
Another alternative is a FAA PPL which only assesses your corrected vision and permits you to fly G registered aircraft.