Haven't been for the class I yet but have discussed my vision at length with the CAA. My shortsightedness is neglible (0.25 R and 0.5L with CAA limits being 5.0) but my astigmartism is about 2.75 in each eye (limit is 2.00).
I've been told I can go for a class I and assuming I'm OK with everything else and the only issue is that slightly higher astigmatism then I'll get a class II. If I get a PPL I can then be treated as renewing my "class I" and will get a Class I (Deviation) upon starting commercial training. Eventually, once I qualify, the deviation will be lifted and I'll have a normal Class I. Bit long-winded and not ideal but there you go...
Mav