I, for one, have not asked British supermarkets, to exercise moral decisions on my behalf.
I prefer to choose myself, whether French wine, Brazilian mangos or chlorinated US Chicken.
Supermarkers are not a relevant gatekeeper, as in you could theoretically (and at least around here in practice) choose to buy at a different supermarket with a different policy. So if some want to boycott brazilian products, let them...
However, states (and their regulatory powers) are relevant in choosing what to allow or not allow on sale. And it is important that they do because without it there is a trap people all into: the downward price-quality spiral.
Mas production lowers costs, and so does mass trade. Sellers would rather not have the overhead of offering many different products if they can sell the same value in just a few. And many buyers prefer cheaper products. Over time this creates a pressure to offer the cheaper options and drop the more expensive ones. This also means offering lower quality products, or less expensive "replacement products". Because sellers won't carry a full range of qualities of products, the better ones tend to be pushed out of sale. And their producers start adapting by lowering or ceasing production. Meaning that the best made products lose economies of scale, get more expensive, and are eventually "nonviable to produce".
In one word,
crapification. To a point where you cease being able to even find a quality product for sale, can only find
crap. Regulation to keep the crap from entering the supply chain and starting this process is a good idea.
Brazilian meat shipments have been known to include spoiled meat... perhaps there are even more direct reasons to put the screws on them that worries about the forest? Unscrupulous people will be unscrupulous in many ways, pressure on brazilian authorities to start
enforcing their own laws are imo welcome. This case is different though, it seems to be about a new law?