Britain need to stay in the EU single market for goods and services, otherwise the economy would risk being at less of an advantage trade-wise, and the effects of this will be further accelerated economically. The UK should be seeking a soft Brexit to at least maintain a good relationship with the EU, and continue the inflow of a necessary skilled and unskilled European labour force to keep the economy afloat to prevent a labour crisis, for example like that of Japan a country with a strict immigration policy. This decision may well deal a powerful blow to the economy so measures should be taken to minimise the negative economic effects of Brexit.
But the issue for the UK is not entirely negative as part of leaving the EU customs union means that Britain will be able to strike its own trade deals outside of the European Economic Area. However, Theresa May's chequers deal is currently lacking a good enough trade policy to be accepted by Brussels, and this is undoubtedly the most important part of negotiations for Britain now, minimising economic damage. Regardless of immigration and how hard of soft a Brexit May pursues, what Britain stands to loose is the access to a good trade relationship with its closest geographical neighbours, and some of the largest markets in the world.