Before the referendum, leave campaigners openly said they wanted to leave the single market:
https://www.bbc.co.uk/news/uk-politi...endum-36270203
Before the referendum, the customs union specifically wasn’t talked about at all, but there was heavy emphasis on the leave side about the EU striking it’s own trade deals and representing itself at the WTO. This, in my opinion, indirectly references the Customs Union.
The Leave campaign rejected a Norway, or a Switzerland style agreement. I understand that a Canada style deal with a free trade agreement was their preferred option.
http://voteleavetakecontrol.org/briefing_trade.html
The above, the UK striking its own trade deals, is
clearly directly related to leaving the customs union.
The UK Treasury wrote a pre-referendum analysis of Brexit, stating:
https://assets.publishing.service.go...ership_web.pdf
http://voteleavetakecontrol.org/briefing_trade.html
The above confirms that the leave campaign based Brexit on leaving the EU political organistaion, and the economic parts of the EU such as the Single Market and Customs Union. Again and again leaving the single market and having an independent trade policy are mentioned.