Legislation against tax avoidance may also, and sometimes has been, retroactive.
Now, I choose to buy my cigarettes at £2.90 a packet including all taxes rather than at £6.50 a packet including taxes. As far as I'm concerned, I'm a tax mitigator. Parliament has made specific laws with conditions I must fulfil; I must be a personal shopper, and the cigarettes must be for my own consumption. HMRC have issued explicit guidance (after being faced with being dragged through the courts) that I may bring into the UK up to 3,200 cigarettes a time without having to justify why. So this is clearly tax mitigation, yes?
Good. Glad that's clear.