Hi Phil M.
Disclaimer: Please understand that I do not claim to be an authority on this subject and you should seek professional advice. However, here is my experience and understanding of the situation.
I was confused by this and may still be! On the CAA web site, they state:
"...the certification limits of eye correction with glasses and the limits of refraction before surgery are the same (+- 5 dioptres), so it is not possible to gain a medical certificate by having refractive surgery, that you would not have obtained by wearing glasses."
I believe that this statement refers to the amount of long or short sight in your prescription, the sphere measurement (first one). If I recall correctly, my eyes meet this requirement but fail to meet the requirement for astigmatism:
"...astigmatism must not exceed 2.0 dioptres."
My left eye's astigmatism (cylinder measurement, second one) is -3.50. However, there is no mention of a pre-operative limit on astigmatism in the CAA Initial Examination Visual Standards pdf, at least not one that I could find.
This lead me to wonder if LASIK would be the ticket to my class 1 medical. I wrote to Aviation House to get clarification in writing. Here's part of their reply:
"...I have discussed your case with Mr Chorley, our Optemetrist here at the CAA, who agrees that should you undergo successful LASIK surgery, you would then fall within the standard for medical certification."
So it would appear that it is possible to obtain a medical by having refractive surgery that you would not have obtained by wearing wearing glasses if the reason you don't meet the visual standards is due to the level of astigmatism and not the amount of long or short sight (refraction?).
Clear as mud?
As it happens, to be safe, I had an "extended opthalmology" at Gatwick just to check for any other reasons why my eyes might fail the medical (there are many) before having LASIK. They found an abnormaility in the endothelium that they suspected was the start of "fuch's distrophy". Having LASIK on an eye in this condition would cause the condition to progress more quickly. I thought at that point that not only would I never be a pilot, but that I might have problems just getting on with everyday life. Next step was a visit to an opthalmic consultant who concluded that this was all a false alarm, so it appears that my flying career is back on. I just need to take the plunge and have LASIK (or some other form of refractive surgery) now.
My advice is to write to Aviation House AMD or even go in and see them. I've heard rumours that the visual requirements might be relaxed in the future too.
Hope this helps and good luck.