Skilled Europeans are leaving in their droves because they are not being made welcome here and they have got the rest of Europe to choose from, so why stay in Britain? If you think that immigration will fall in the long run, that's not likely. The government may talk tough on immigration, yet they know it is essential because of the ageing population. At the moment, Britain currently imposes very strict restrictions on immigration from outside the EU, but if Europeans no longer come, the immigration will have to come from places that Brexiteers approve of even less, such as Africa and Asia.
If indeed the British economy does falter, due to the inability to attract skilled migrants because they prefer other European countries, the British unemployed will no longer have the option to work in Europe like the tradespeople in "Auf Wiedersehen, Pet".
There is also a comprehensive motorway network west of the Pennines.
Economic success doesn't necessarily follow from road building.