There is a difference between tax avoidance an tax evasion. Tax evasion is unlawful, tax avoidance is working within the law to minimise tax liability.
I should caveat this post with the fact that I am employed and all my income is taxed through PAYE. If I had more sources of income and more of it, I would not object to paying tax on all of it (by way of example, I never seek to reduce payments by paying cash to plumbers etc.), but equally, I would not want to pay more tax than I had to in order to remain on the right side of the law.
It seems to me that for wealthy individuals, paying an accountant to advise on minimising your tax liability is perfectly reasonable. If the government doesn't like the net effect of this, then it is the job of parliament to pass legislation that closes the loopholes that permit the "avoidance". So why is David Cameron bleating on about tax avoidance as though it were some sort of criminal offence?