Yes the market is the upside.
The downsides are:
-death to business by regulations
-self-implosion of competitiveness due to shutting down of reliable, cheap, and safe fossil fuel energy and nuclear plants
-increase in family energy poverty due to same reason above
-capital misallocation by central planners
-warmonger bureaucrats whose spending and political decisions impoverish normal European families
-authoritarianism in shutting down democratic elections, or preventing elections on key issues
-unpopular mass immigration policies of a great deal of people who do not assimilate in Europe.
At this time, the UK does not seem to be taking any different decisions than the EU. If anything, UK might be worse on many of the above points.
Although it is possible that the UK could take different decisions, as they salvaged greater independence from Brussels.
