Trawled the interweb last night and came up with a solution for the ios 6.13 greyed out wifi toggle ........ and it works
Download "any" torch application, you need this to rapidly drain your battery
turn it on and allow the battery to totally run flat, leave it off for about 2 hours
turn it back on by charging ...... dont play with it, just leave it to charge.
after fully charged, turn it back on and go to "settings, wifi ..... and hey presto you have wifi again.
Dont ask me how this works, but the only thing I can come up with is the heat this produces when discharging your battery might reseat anything. and not touching it allows things to cool down and settle in place.
All I can say is it worked for me....... so I saved myself money as apple would surely have charged me for a new phone rather than look at it.