Well, no, actually. It's not hard to look at the expected effects of brexit and examine whether they are occurring. For example, decline in small forms exporting to Europe due to trades friction.
"But the working assumption of the Office for Budget Responsibility (OBR), the government's independent official forecaster, is still that Brexit in the long-term will reduce exports and imports of goods and services by 15% relative to otherwise. It has held this view since 2016, including under the previous Government."
"And the OBR's other working assumption is that the fall in trade relative to otherwise will reduce the long-term size of the UK economy by around 4% relative to otherwise, equivalent to roughly £100bn in today's money."
https://www.bbc.com/news/articles/cdrynjz1glpo
Play stupid games, win stupid prizes.