A flat taxation system (with say the starting point for tax at £25,000) seems like a good plan? I also think taxes on business and taxes on individuals should also be harmonised. Add in a general tax avoidance rule and you are good to go.
Generally, tax evasion is a crime that can and should be prosecuted. Everyone agrees with that.
Tax avoidance, or tax planning, is in Wikipedia terms "the legal usage of the tax regime to one's own advantage to reduce the amount of tax that is payable by means that are within the law."
So, tax avoidance is the kind of behaviour that is explicitly and clearly legal. Now someone wants to make a "rule", apparently to somehow punish for it?
Of course there are loopholes and quirks in any tax system, and people are exploiting them. Particularly the rich. But the solution is to close loopholes and make the tax system simpler and clearer, not have a "general tax avoidance rule" which I take to mean arbitrary confiscation and punishment according to political whims.