The question is, "What single thing would have the most positive effect of any on this nation?"
Not too easy, is it?
Even so my answer would have to be to see the raising up of a new Wesley.
Wesley evangelised an entire country and transformed it by bringing Christ to the masses who were only peripherally involved in the C of E (of which he remained a member his entire life.)
In practice, such a man today would have to have an even greater anointing than Wesley. Matters have become rather more difficult in modern times.
1) In the eighteenth century, at least Man had some sort of concept of God. A good starting point.
2) The great evangeliser would have to overcome decades of neglect of The Almighty and The Scriptures.
3) He/she would have to overcome years of inbuilt atheism and negativity throughout the educational system and the media.
Yes, indeed. That is who or what we need - an itinerant preacher of massive gifting from God.
Perhaps it is past time for Christians to make this a specific target of prayer.
But then - why ask for just one?