Tax avoidance = doing legal things to minimize taxes
They're different. It may sound like a small difference, one that we can ignore in our dudgeon, but it is a rather large, rather principled difference.
I claim my one child as a dependent; he's underage, so I get a tax credit; he was in daycare, so I get child-care credit. This is tax avoidance. I am avoiding paying taxes that I would otherwise be subject to.
I do not claim my wife's cats as dependents, nor my son's cockroaches. That would be tax evasion.
Similarly, my wife and I carpool to save on gasoline.
We could also save on gasoline by stealing our neighbors car and using that to commute to work, or stealing from a local gas station. One's legal. One's not.
We confuse "we think this is immoral" with "the statutes say this is illegal." We can do this out of ignorance, we can do this because we think we're omnipotent and are the collective Decider of Laws of Statutes, a sort of DUcracy, or we can just do it because we think it sounds Really Cool to Be Outraged and take strong, unprincipled positions.
If we want things that are legal to be illegal, we have a way of bringing society in line with our values and morality, presuming that we either have sufficient ammunition or sufficient votes and compliance from other, lesser, citizens who have no rights. It works the same in the UK.