If the EU sticks to its view that it cannot allow the UK to have any control over migration, the negotiations will be very short. The UK must refuse to continue with freedom of movement, so there will be no basis to reach a new agreement.
This means the Uk will simply need to withdraw from the EU and rely for our trade with the EU on most favoured nation status under World Trade Organisation rules. Average tariffs are very low, though they can be higher against German cars and French agricultural produce. The UK does not want to impose tariffs, but if the rest of the EU does impose tariffs up to WTO limits the UK will obviously retaliate.
The UK could make a good living under WTO rules. The UK will still benefit from the main advantage of the single market, the fact that you can produce a product to the same standard to sell anywhere in the EU. The recent fall in the pound is bigger than the extra costs WTO tariffs and ruled could impose, so our competitiveness will still be better.
In some ways this makes it so much easier. If The EU does not want to listen to the UKs needs then they have to accept we can just leave and they may end up imposing obstacles to their very successful exports at a time when the fall in the pound has just made them dearer.