As far as I'm aware and understand the situation, a company supposedly based in a "Tax Haven" might buy/sell/make a lotta profit by doing so in the UK but because their HQ is supposedly in the tax haven, they don't pay tax here, they pay at the rate set in tax haven which is usually very low compared to the UK.

The tax havens are usually "poorer" countries which try to attract companies to "base" themselves there so the tax haven earns some tax which it otherwise wouldn't have done if they hadn't "attracted" companies by charging such a low rate of tax.

For a company, its far cheaper doing all that than paying UK corporation tax. The shareholders also benefit because their "earnings" were "earned" in the tax haven where the rate is low.

Now DC sold his shares in Blairholdings but because he "earned" that profit in the UK, he paid UK tax on it.