X - Tax Avoidance and Evasion:
* The UK Government estimates that a minimum of 31,000,000,000 GBPs is lost in tax revenue each year by deliberate evasion and avoidance
Tax avoidance is legal, and the government encourages it by (for example) offering ISAs, abolishing tax on interest received etc.
Tax evasion is not legal, but the current thinking at HMRC is that by doing deals with individuals, they can get some tax whereas if they pursued these people for everything that MAY be owed, they'd get nothing.