Welsh lamb only becomes uneconomic in the EU, it's major export market, because of brexit and the subsequent tarrifs that will need to be imposed.
You really aren't following this at all.
Why is Welsh lamb uneconomic in Europe because of a 40% tariff that has to also be paid on lamb from New Zealand, Australia and South America?
That just makes an even playing field.
Now, if you're saying that Welsh farmers have gotten sloppy in their business practices because their products didn't have the 40% tariff before, and now will be uncompetitive because they lost the competitive edge, you might be right. So what they'll have to do is sharpen their business practices so they can compete with others like Australia and New Zealand. After all, nothing else sharpens a person's attention like the hangman's noose.