I would asssume that the £30k without paying income tax

  Very comfortable life: £30k per year.  Which still corresponds to £40k gross salary (roughly 1.5x statistical median wage), but with tax-efficient asset allocation you can relatively easily achieve £30k per year without paying any income taxes

comes from drawing down savings rather than earning income? At £40k annual income, your effective tax rate is 14.7%[0], so without complicated self-employed tax reporting (deducting most of your expenses), I don't see how you can reduce taxes on earned income. Sure, the ISAs (read: tax-free savings accounts) give you tax-free earnings, but they haven't been around long enough for you to make all of your income from them[1].

0 - https://www.gov.uk/income-tax-rates/current-rates-and-allowa...

1 - https://en.wikipedia.org/wiki/Individual_Savings_Account#Ori...