The CAA recommend against varifocals - trying out a mockup in the opticians made me feel sick very quickly. I use bifocals built especially for flying, with the near vision part optimised for reading the map on my lap (I took the map in to the eye test) and my normal distance vision prescription. It turns out that I can read the instruments through either part of the lens.
And of course the necessary identical spare pair. Total around £500, but who said flying was supposed to be cheap? (Actually I don't just use the bifocals for flying, I've discovered they're quite good for chairing public meetings too, as I can both read the papers on the table in front of me and recognise which punter is waving at me from the back of the hall.)