And then, in the wake of the tax haven scandals around 2005/6, the HMRC wrote me a nice letter explaining that, while I had obeyed all the rules at the time, they were retroactively changing the rules and I needed to declare all the interest I had earned on my offshore account over the last 8 years. The final bill came to about fifty quid, so no major damage done. But the lesson has stuck with me.
HMRC can and will change the rules at any time, and apply them retroactively. Getting super smart about taxes, especially by using loopholes and tax havens, can be counter-productive. Unexpected tax demands are not fun.