The economics of a "no deal" Brexit A "no deal" Brexit is presented in negative terms.

In such a scenario the UK would set tariff schedules in line with trade strategy and protect domestic industry/agriculture with tariff rate quotas.

The UK would lose tariff-free exports into the EU and the markets with which the EU currently has free trade agreements (the two main ones being Japan and Canada). 

The non-weighted average MFN global tariff is 9%. The weighted average tariff for UK exports into the EU is estimated to be around 4.5% in this scenario. Natural movements in currency value are of this order of magnitude.

There would be increased trade friction for the 44% of our exports that are sent to the EU. British exporters would have to follow the customs rules used by other 3rd-country exporters to the EU like China and the USA.

This would be a major change. Most countries export to the EU under these terms. Our reliance on services mean this change would be economically bounded.

The productive base of the UK would re-orient around the new status-quo with some sectors decreasing in competitiveness and diminishing and some increasing and expanding. 

The overall success/failure of the new status quo would be determined by the wider economic strategy rather than tariff changes that are mostly small.

Why is a negative long-term forecast necessitated?
Maybe someone can get into more details, but generally speaking


  The range of estimates is large, from a loss of GDP of nearly ten percentage points (in the least
  attractive trade and inward investment scenarios modelled by the Treasury, NIESR and the Centre
  for Economic Performance at LSE)
  1
  to a gain of four points (Minford, for Economists for Brexit
  – a
  clear outlier) – see figure 1.
  
  
  
  The main reason for the differences lies in the assumptions made about shifting from the
  current access that the UK has to the EU single market to a new regime in which the UK
  faces restrictions. 


Those were from 2016 estimates/papers.

Also, economists were not terribly good at predicting Brexit-related effects insofar, at least short term; see https://economics.stackexchange.com/a/27492/6210 While the gloom and doom may have been overstated, some effects are apparent already, in some analyses:



Apparently the chilling effect of Brexit on investments may have been a real thing (but that assumes the counterfactual dotted trend was a sure thing).



As a sort of an update, some days ago (Sep 3, 2019) UNCTAD has posted an article on the narrower issue of potential UK losses in a "no deal" scenario due to continuity trade agreements with other countries not yet agreed.


  In relation to exports to countries with which the UK has not yet secured the continuation of preferential access, the UK could face substantially higher MFN tariffs, especially on agricultural products. For example, processed food products could face an average increase in tariffs of about 17 percent. Similarly, apparel, textiles and motor vehicles would also face substantially higher tariffs. In value, most of the losses would be concentrated in motor vehicles (about USD\$750 million) and in chemicals (USD\$200 million).
  
  [...]
  
  Among the countries which currently grant preferences to the EU but for which the UK has not yet reached an agreement to grant continued preferential market access, Turkey, South Africa, Canada,  Mexico, Japan, Egypt and Morocco are the markets where the UK is expected to have larger export losses. In particular, the UK is expected to lose about USD\$500 million  of exports in the Turkey market, about 5 percent of its exports to Turkey. In South Africa, the UK is expected to lose about USD\$240 million, equivalent to about 9 percent of its exports to South Africa.


There's an update to the update as well. On Sep 11, the UK has announced an "agreement in principle" with South Africa and with a few other, smaller countries from Africa.