England's GDP has historically grown at 1.5-2.0%. That implies about 20% growth in a decade.
Let's say Brexit leaves GDP 2% lower than it currently is by the end of 2023. Now to grow 22% in 9 years (i.e. get back to even), they need 2.4% annual growth, which is a 20-60% acceleration sustained every year for 9 years just to get back to the baseline.
Less GDP today, in many ways, is less GDP forever.
Small base growth is easier (that’s the example you give), but catch-up growth in large economies is damn near impossible.
This may be interesting to you: https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)...
Your 40% gap turns into an 18% gap. Further more libertarian economies don't look particularly exceptional.
Together this suggest that the differences in GDP are much more easily explained by existing wealth and foreign policy/trade, instead of domestic factors.
I also understand that certain policies can lead to changes that will disturb QOL in a country outside of raw GDP (go ask Swedes how they feel about immigrant behaviour in their country in recent years). Is Swedish GDP up? Absolutely. Is it worth the price of open hang warfare and frequent shootings that have sky-rocketed since 2015? Depends how you look at it but this isn't something that shows up in GDP but has tangible impacts on people's day to day lives.
GDP growth isn't some static figure that will permanently remain low in the UK due to Brexit. There are many tangible real projects that could lead to significant improvements in GDP growth and QOL improvements that have nothing to do with Brexit (transport infrastructure improvement, housing reform, NHS reform etc).
Brexit happened. Maybe it shouldn't have, but it did. The last thing the UK should do is try and rejoin now at a point of weakness especially when things aren't exactly looking wonderful in much of the EU either. Better to engage the rest of the world where possible, spend some time and effort focusing seriously on internal development (including measures to shore up the union so that Scotland won't be driven to make an incredibly self-destructive decision with independence) and then reopen negotiations when the UK is in a better spot.
Despite the doom and gloom we are still talking about a top 10 world economy with world class human capital in many industries, that speaks the global lingua franca, has extensive overseas ties, and a lot of projects that would clearly help improve QOL in the UK. Focus on that first instead of trying to call back a ship that has already sailed.
Why though? To save face? What if both the EU and UK would still do better together than apart? At the same time, why would Scotland want to be tied to England who isn't doing so hot?
On top of that one data point is 19% of Scottish exports went to the EU. 60% go to the rest of the UK. Scotland has no ties with any foreign countries (they all run through Westminster), a fraction of the population that would struggle to achieve economies of scale, etc. On top of that there isn't even any guarantee that Scotland would get into the EU (I imagine Spain would block it so as not to encourage Catalonia separatism - check the 2017 Catalan referendum for my point).
Even if you think the UK leaving the EU was the UK shooting itself in the foot, Scotland exiting the UK would be it shooting itself in the head.
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In terms of rejoining the EU it won't even be able to brought to ballot for at least a decade. All the remain campaign would need to do is start running ads about how EU law will supersede British Law and highlight that the deal the Brits are getting now isn't as good as they one they had (and imply this is being done for vengeful reasons) and watch support collapse. The UK should be aiming for a free trade and economic agreement, not EU membership. That ship has sailed.
But that doesn’t mean the original decision has benefits.
Why do they need to get back to even? Why is this assumed to be a realistic goal in the first place? This seems to assume the rest of the EU is going to be just fine.
The only things that matter are if the UK will do better or worse than it would have if Brexit hadn't happened, and how they'll do compared to the EU.
Before, Brexit looked pretty stupid, and the rest of the EU looked like they were in a much better position. This is no longer true; maybe the Brexiteers will turn out to have been correct after all. Only time will tell.