A couple of months after being diagnosed I had an optician refuse (very politely) to test my eyes until my blood sugars had stabilised.
If there's any choice in where you go, the place to avoid is begins and ends with "S", has three syllables and is convinced that they can save you money on your specs... I can see better without my distance glasses than I can with them. In fact I'd probably fail the "read that numberplate" part of the driving test with the glasses on!
As a matter of interest what documentation to opticians need to re diabetes? Is it enough to show them a pack of, say, gliclazide, with your name on it, or is there a form that needs to be got from somewhere awkward to get to?
Steve