Trade with Argentina is very small and Argentina has a trade surplus with the UK, before we joined the EU, Argentina not Ireland supplied the majority of imported beef to the UK. The EU has just increased quotas of beef from Argentina, they of course would like to get back to pre 1972 levels of beef exports to UK. No present trade will be disrupted while this is negotiated.
The present trade with the 24 will carry on as before, but what will replace the loss of the 68 WTO EU affiliates and what will replace the EU business? The claim that these losses can be easily be renegotiated is nonesense. It will take many years of renegotiation. Meanwhile, the 24 countries with whom we have current trade deals will agitate for an advantage, knowing that the UK is in a position of weakness.
The UK produces 75% of its own beef and of the 25% that is imported, 70% comes from Ireland and the nearly all rest from the Netherlands, France, Poland and Germany. Without a deal, what happens to those 25% of imports of beef is uncertain. Argentina, as you say, want to increase their sales but the USA, who are also one of the 24, will also want to sell more beef and beef products. The USA put a ban on british beef in 1998 due to the BSE crisis and we reciprocted by banning US beef. In 2016 the USA and UK signed an agreement to lift the ban due to start in 2018.
The USA currently claim they are being discriminated against because of the EU wide ban on hormone impregnated beef and cloned beef products. When we are out of the EU, the USA will push for those to be part of an agreement and Argentina will compete by selling 'clean beef'. If the UK decides on one or the other, the losing competitor will claim discrimination and the whole thing will end up in a WTO court taking years.
The problem for british beef producers is that the hormone impregnated beef is from cattle reared with growth hormone. Per kilogram, it is cheaper to produce. They will face cheaper competition from the USA and have no option but to lobby the government to allow the practice in the UK. If we don't want to go down that line but still allow imports from the USA some of our beef farmers will go out of business. If we ban those imports, the USA will take us to a WTO Court. Argentina of course will look for a position to take avantage of. If we allow the practice, the british beef exports will suffer as many other countries do not allow it to be imported.
It's the same situation with AquAdvantage Salmon from Canada. It is genetically modified farmed salmon to grow twice as fast and twice as big as conventionally
farmed salmon. Scottish salmon farmers will face a situation similar to the beef farmers and lobby the government to ban those imports. If they do, Canada are sure to object. It is all time consuming with uncertain outcomes and not the easy step onto WTO trading with things carrying on as normal as many people claim.