Tax evasion is lying in order to reduce your tax burden. Tax avoidance is following the letter but not spirit of tax laws. The lawyers behind those schemes claim they're legal, but it's important to remember that tax authorities often close schemes and claw back the avoided tax. UK tax authorities tend to avoid going to court if possible so a lot of tax avoidance actually probably is illegal, it just hasn't been tested in the courts.

You're mistaken about tax avoidance being a duty of management by the way. "Fiduciary Duty" doesn't give companies a free pass to be assholes. http://www.washingtonpost.com/blogs/wonkblog/wp/2013/09/09/h...