TB T, I think they add half the cylindrical component to the spherical and then look at the difference, so you would be ok on that basis.
Much simpler is to telephone the medical department at Gatwick and ask to speak to them. They can be very helpful, and don't even ask your name. It would probably help if you faxed them a copy of your glasses prescription.
If you want a class one medical they will even arrange it so you just pay for the eyetest part £49??? to see if you pass that rather than pay for the whole £500 ? worth. If you pass the "eyes", then you can let them probe the rest of your body for signs of decay and debauchery .
Good Luck !