There are other issues than just the quality and standards of imported Australian meat.
If the UK government allows the import of meat and agricultural products from Australia that are produced to lower standards, then every other country will demand this as part of a trade deal. Particularly the USA, Canada and Argentina.
Importing food from countries 10,000 miles away rather than from countries just 22 miles away makes a huge difference to the food miles and associated pollution. It is only financially viable as long as ships are allowed to burn low-quality bunker fuel. If this is outlawed then the cost of freight increases as fuel prices increase and the number of available ships reduces.
While the UK is not self-sufficient in food (and hasn't been for 170 years) it is a dangerous policy to become almost entirely dependent on imported food, which would be the case if British farms were to be forced out of business. What happens when food prices rise, when there is a crop failure or disease in a major food-producing country, or when conflicts (or a ship stuck in the Suez Canal) disrupt supplies?
Finally, farming is still an important employer in the UK. Not just the farmers, but also the associated industries (vets, food processors, agricultural machinery companies, transport companies etc) employ many more people than the industries that Boris is so keen to promote. Whole communities would collapse if farming were to become uneconomic - which is one reason why governments everywhere are keen to subsidise it.