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 and address any labour shortages. This decision may well deal a powerful blow to the economy, so measures should be taken to minimise the negative economic effects of Brexit.
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, in order to minimise economic damage. The main problem is with customs and trade, May's Chequers deal is seen as asking to pursue an independent trade policy while retaining all the benefits of formerly being an EU member. The EU thinks this is unworkable as other nations may pursue a similar exit-with-benefits strategy.
May has repeatedly delayed the House of Commons vote for fear of rejection of her exit plan. Something that became painfully true on 15th January when parliament overwhelmingly rejected her plan. Many other policy issues remain vague, with indefinite dates or even unaddressed completely as the government continue to discuss the destabilising potential of a re-vote, the possibility of revoking article 50 and the grave danger of not making a deal. It is now urgently important the a "plan B" goes to vote soon, as the longer she holds out the less time there will be to make a deal and the more likely a “no-deal” Brexit looks.
Regardless of immigration, or how hard or soft a Brexit May pursues, what Britain stands to lose is the access to a good trade relationship with its closest geographical neighbours, and some of the largest markets in the world, so some deal is at least better than none. But as the governments indecision makes the reality of a no-deal Brexit seem ever more present the implications of such an exit from the EU, considering the large amount of world financial services based in London, could have big repercussions for markets worldwide. However the short term economic predictions were wrong and with the right move swiftly made before the 29th March deadline, maybe the long term economic impact could be turned around too.