I can't speak for the merits of legal case against the Brexit campaign. In any case, it is a waste of time and would deliver at best a moral victory - in the unlikely event it even got to a conclusion.
Perhaps think the money should either be returned to the donors, or if they agree it could be put into a trust fund of some sort which may be the basis for a campaign to rejoin the EU in a few decades.
But I have to say I am in awe of the chutzpah of the Brexit campaign.
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