Moonie, tax evasion is illegal. That's where someone has deliberately not informed HMRC that they've been getting money which they know they should have paid tax but haven't.
Tax avoidance is where someone has done something to save tax. It is a much wider spectrum than evasion. At one end of the spectrum is putting money into an ISA account so that no tax is due. That's above board, and there's nothing immoral about it. It can be described as tax mitigation which is that part of tax avoidance which is completely legit.
At the other end of tax avoidance are these artificial schemes which make use of loopholes in the tax law. They may be legal but many would consider to be immoral. These are the schemes which get into the news.
I don't get involved with those sort of schemes and don't have clients who do.