Now, is any leaver prepared to admit they might be wrong and Brexit could turn out to be one of the worst things Britain has ever done?

My vote may have been cast for the wrong option. There is always a possibility that Brexit will not be of long term benefit to the UK, but I don't believe that will be the case.

As to "one of the worst things Britain has ever done", what are we comparing it with?
Empire building
Slavery
Trying to quell uprisings in our colonies (including America)
Attacking Germany
Turning Australia into a penal colony
Inventing sports so that other countries can feel superior to us
There's so much to choose from.