A distinction between mitigation and avoidance? I'm not sure really, I guess you could argue that you mitigate your tax bill by using all available reliefs, allowances etc, whereas avoidance would be a deliberate action to take a piece of legislation and bend it to your advantage and away from it's original purpose.
However I suspect those involved in avoidance would say they are mitigating their tax bill
The big distinction though is definately avoidance and evasion. I'm not sure what can be done, on top of what the government is already doing, to stop either one.
If attitudes were to change and it was seen as a bad thing to not contribute what is rightly due (which of course some of us do believe) then that may mean more tax would be declared correctly. Indeed for many large corporations, as well as honest singular taxpayers, they do want to be known as fair, honest companies and so would definately steer clear of any negative activity. And I think most people are inherently honest, regardless of whether we are discussing tax or benefits.
I do think some are inherently dishonest and some will try and see what they can get away with. Those are the ones that are targetted.
So what do you think could be done differently to decrease the amount of tax lost?
TSC you mention that someone was employed by her husband but didn't work for him to get tax breaks. I assume what you mean is that she was paid a wage without doing any work to use up her personal allowance? That would be tax evasion as it is a false situation created to minimise tax paid as a couple. But a family not working more hours is a decision made within the law by them to maximise the amount of money available to them. Which I think says more about givernment incentives than a family trying to get by.