I wanted to find out if you fully understood the figures and what they represent, because I remember the article you are referring to. It is very misleading, not to mention extremely ambitious as regards what it believes we have missed out on in economic growth over the last 3 years due to uncertainty over Brexit.
The majority of people reading that article, and also reading your post, would assume that what the Government has lost in income from economic growth over the last 3 years is roughly the equivalent of what we have paid into the EU since we joined. Yes? Well actually No. What the article is saying is that the UK has lost out on economic growth of £130bn over the last 3 years. Even if these figures were remotely accurate, the UK governments annual income is roughly about 35% of GDP, so it's income from this "missing" economic growth would only be in the region of £45bn. Compare this to our total "net" contributions to the EU over the last 47 years of around £177bn. By my reckoning that's just over 25%.
Then you ask the question where do they get these figures for loss of growth from and how do they justify them. Just looking at the economic growth rates for 2018 and 2019, the UK grew by 1.3% and 1.4% respectively. During the same time period Germany grew by 1.3% and 0.6%, the USA 2.9% and 2.1%, and France 1.7% and 1.1%. Now if we assume we are missing out on around £45bn growth (£130bn divided by 3) per annum, that would actually increase our annual growth three fold. So our growth would actually be around 4%. It goes on to say that we will lose an estimated £70bn growth this year too which would have our growth rates pushing 6%. How can they justify these sort of figures when our peers are performing much the same as us, if not worse.
There is no doubt that our economy has been held back to a degree due to the uncertainty over Brexit, but the figures being quoted here are much more pie in the sky than the future trade deals you mention, most of which we haven't even begun to negotiate yet.
This is exactly what I refer to when I talk about scaremongering and "project fear".