I think its just the carbon costs associated with growing and transporting all that grain which grass-fed cows don't have. Grass-fed also self fertilise, with no transport costs! Grass-fed free-range beef is said to be more or less carbon neutral. Grain-fed on the other hand adds significant carbon to the environment.
Obviously transporting beef all the way from Oz can't be good either.
Suffice to say that for food producers and for UK food consumers, Brexit hasn't been a great success so far. It has increased costs and complexity of buying/selling for and from local producers and lowered standards for imports. Maybe there will be some lower prices for certain items like grain-fed beef in the future, but that is hardly a win. Hopefully we'll see more British-produced foods consumed in the UK. Things like Markel, Lamb etc are great value produced on our doorstep. Undermining our own producers and buying cheap lamb and beef from the other side of the world (which is lower quality) doesn't seem to make sense. Most were expecting more from Brexit, weren't they?