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.
The problem is that the avoidance schemes are often down to interpretation and it often takes an expensive test case in the courts to get a ruling on the legality. I believe the Gary Barlow/ Jimmy Carr cases were more about their public image and if they weren't "celebraties" they may not have apologised quite so quickly. I doubt either of them really understood the advice they were given by their expert advisors.
The morality of tax avoidance is another issue of course.
