The EU is the UK's most important trading partner, responsible for half of British imports and exports. It allows for the lack of trade barriers thanks to the free flow of goods and services. After leaving the EU, the conditions of trade will deteriorate and its size will decrease.
For consumers - prices of goods imported from the EU will increase, for businesses - export will become much more difficult and more expensive (customs, border controls, differences in regulations, inability to benefit from preferential trade conditions that the EU negotiates with other countries, such as the US or Japan)
Great Britain is a country where foreign companies have so far been keen to invest. The inflow of such investments (so-called FDI - foreign direct investments) will be severely restricted, as foreign companies manufacturing in the UK will no longer be able to freely sell their goods and services across the Union, while the British currency may turn out to be less stable. The fall in these investments is another blow to the economy.
The decline in trade and investment also means fewer jobs. Uncertainty about employment opportunities will mean a lower propensity to consume which will also affect Gross Domestic Product (GDP) growth.
Immigrants do a lot of work which the British themselves do not decide on. They pay more taxes and contributions than they receive benefits, thereby improving the condition of British public finances. Closing the boundaries against them will deepen fiscal problems.
London City will lose its status as the financial capital of Europe (8% of British GDP), and many stock exchanges, banks and funds will probably move to other locations.