Plenty of other places to export stuff outside the EU, which makes compliance optional.
[0] https://en.wikipedia.org/wiki/List_of_the_largest_trading_pa...
The question you posed assumes that the EU will continue to be involved with half of the UK's trading activity, which I don't buy.
The difficult bit to buy, hard as Brexiters have been selling it, is that the reduction in the proportion of trading activity with the EU will be compensated for by an increase in trading activity with countries which are further away, more protectionist and/or poorer, as opposed to just a reduction in trade overall.
Regulations, imports from the EU, taxes paid to the EU, and tourists to and from the EU are also part of the equation. So the UK might be better off outside the EU when you ignore trade.
Having said that, offsetting some of the drop in EU exports with and increase in non EU exports seems likely and could allow for a net benefit even assuming a net drop in total exports.
We've had several years to hear a concrete argument for what regulations need removing and what the benefits would be, and the best we got was Patrick Minford's ambitious "it's likely we would eliminate manufacturing" growth projections based on the assumption the UK removed all barriers to importing anything. Ironically the EU has made more progress in securing FTAs with our list of putative alternative trade partners...
It’s possible for the increase to exist, it to be less than the loss, and for the UK to be better off. As such some people talking about the likely increase may not be implying it’s a 1:1 replacement for the drop.
Generally being part of a larger market is the best option, but individual executions are clearly going to be argued about endlessly.
Those sectors could always have chosen to abandon trading with the EU to find better arrangements.
post-brexit: the rest of the world will be on a more even footing, so this will likely change
You can look stuff up here. Lots of things are, as you'd expect, reduced but some things surprisingly high still.
https://en.m.wikipedia.org/wiki/Trade_agreements_of_the_Unit...
Remember though that an average trade deal takes longer to negotiate than the time we have legally had to negotiate them so far. It is likely that in the next 5 or so years we will get an agreement with the USA, CPTPP and India.
[1] https://commonslibrary.parliament.uk/research-briefings/cbp-...
To use an extreme example, if the UK suddenly decides to legalize pot they could then export that while continuing to export every other product under EU regulation. Net result an increase in exports.
The whole goal of brexit seems to be to lower U.K. food standards to that of the US.
> As an EU member, the UK was automatically part of about 40 trade deals which the EU had with more than 70 countries. In 2018, these deals represented about 11% of total UK trade.
> So far, 29 of these existing deals, covering 58 countries or territories, have been rolled over and will start on 1 January 2021. The latest deal to be struck was with Mexico on 15 December.