One of the big problems in the UK is that there is a huge disparity between enforcement on two groups.

Group 1

Typical individuals, SMEs, etc.

and

Group 2

Large corporations, politically and otherwise connected individuals

Group 1 come under enormous scrutiny and given the powers that the revenue has this can be a ruinous process, which we all fear. (Thus, there's a large business in selling "audit insurance" to SME's to cover the business disaster that is a Revenue audit.)

Group 2 get very little real scrutiny.
It's partially just that the richer you are, the more you can afford fancy tax lawyers to find loopholes in the first place and partly a matter of political will.
But a big issue is simply that compared to a large transnational, the Revenue is up against it. Some big corporation (e.g. Vauxhall/GM at one point) employed more tax experts and lawyers than the Inland Revenue had in the whole region where the headquarters were.