Experts need a coordinated and stable environment to do their thing.
I think that IF the UK would be able to use her experts in a normal fashion, the scenario it can achieve will be similar to Japan. Stagnated population, stagnated GDP, stagnated trade since decades now.
Consider that Japan is in a similar basic position as the Brexit UK is heading for: a high developed country with little trade with nearby countries.
Without immigration the population growth of UK will be around zero.
Without increasing trade as % of GDP, GDP will stagnate like Japan, unless the UK can grow relevantly needed economical sectors to a competitive level, for a more wholesome economy like for example the US.
If hard Brexit would decrease trade as % of GDP from 60% to the 30-35% of US, Japan etc..... and assuming that the rule of thumb applies that you get 3% GDP growth per 10% "trade as % of GDP" growth... GDP would go down with more than 6%.
Improving the quality of the domestic economy, aiming at the right diversification, priorities and competitiveness, the way to go.
A lot of experts and good coordination between politicians, experts and what the people are willing to accept mandatory.
Still doable over time. That recovery of that 6+ % being doable. After that stagnation like in Japan is again the most likely outlook. After all, the US is of a much bigger scale size for a high level "stand alone" economy.
But I am afraid this Brexit move is for now under control of a ruling elite that will more likely use eroding of costly social and health security, etc as the quick fixes for anything not achieved by the hard work of building up a healthy "more stand alone" economy.
Even if the Labour party would govern a bit more than 50% of the time, that will not change that social eroding in a substantial way. There are too many complicating factors, domestic and global, that play in the hands of the neo-liberals for the coming time.