Whatever site you’ve just used, it’s wrong.
You don’t pay £5,000 in basic tax plus national insurance on a £25k annual wage.
That means that person has no personal allowance.
Earn £25,000 gross in 2019/2020 and you'll take home £20,538. This means £1,711 in your pocket a month.
Over the year you'll pay £2,498 income tax and £1,964 in National Insurance.
ie your total tax would be circa 20% (if we call NI a tax, which it isn’t, in terms of basic income taxation, it’s contribution towards your state pension qualification and the NHS)
That’s why I’m asking if this 80% thing is then taxed. Because if it’s not, it’s basically the same net pay people would have received before.