You've misread the sentence. I'll grant it's not clear, but this sentence follows the fairly common British form of "legal methods" meaning "acts involving lawy

You've misread the sentence. I'll grant it's not clear, but this sentence follows the fairly common British form of "legal methods" meaning "acts involving lawyers", not "acts which are not illegal". So this BBC article is simply stating that various companies have been accused of engaging in legalistic and accounting techniques to minimise their tax bills. Nothing is suggested about whether or not those techniques are illegal. Tax evasion is certainly illegal though!