//So, in that way Brexit has been fermenting for forty some years.//
Indeed it has.
The UK has never been an unconditional supporter of the European Project. We joined principally for trading reasons (and those reasons were very valid and laudable – and still are). But it never subscribed to the federal ambitions of the Euromaniacs. There are fundamental differences between the psyche of UK citizens and those of our continental neighbours. The “Common Market” was born when national leaders were fearful of their electorates, remembering that Germany had only comparatively recently voted in a certain Herr Hitler. Those democratic freedoms had to be trammelled for fear of a repetition. What better way than to slowly erode the powers of democratically elected Parliaments and governments and transfer those powers to a self-selecting cabal of bureaucrats over which the electorate had no control?
So the seeds of Brexit were always in the ground. They just needed a drop of rain to make them germinate. That drop came in 1992 when that Nice Mr Major signed the Maastricht Treaty. This effectively set Europe on the road to a unified federal state. The plant flourished, being nourished by the introduction of the single currency and the Schengen agreement and a couple of intermediate (salami slice) treaties of Amsterdam and Nice. Finally in 2007 it burst into full bloom when the Treaty of Lisbon was foisted on the member nations (resistance from those countries fortunate to have had a vote on the matter either being ignored or crushed by the imposition of second votes). This effectively established the EU as a sovereign enterprise: the unanimity previously required on major policy decisions was replaced by majority voting; posts of long term President and a “Foreign Minister” for the EU were established; the EU announced itself to the rest of the world as a single legal entity.
So Brexit is not a modern phenomenon. It has been bubbling under for decades and would have burst to the surface at the first opportunity, almost certainly if that opportunity came after 1992 and about as definite as anything can be had it come, as it did, after 2007. It’s doubtful Mr Johnson had much to do with planting those seeds or germinating them. The same thing would have happened without him because the EU and its predecessors have so many deficiencies almost anybody could have highlighted them.