It also effects your eligibility for free pre-school child care, which is worth £9000 /year to effected people. As you are paying 50% ni and income tax from 80 to 100, you are better off earning 80k than 100k.

You can get around this with salary sacrifice pension, which can push your pre tax income down bellow the critical levels. But most employers refuse to offer it.

At a small company, you might be able to make them do it.