My, perhaps simplistic, understanding has always been that tax avoidance was reducing the amount of tax paid using legal loopholes (which HMRC at continuously trying to plug), tax evasion was reducing the amount of tax paid by deliberately mis-declaring income, assets etc.
I find the best way to remember which is which is to use a Star Wars device. Darth Voidance isn't good, but the evil one is Darth Vader.
