It's generally a good idea to check them yourself as well. HMRC does sometimes make mistakes with people's tax codes, resulting in the wrong amount being taken off your pay, and your employer doesn't (and can't) correct those mistakes for you. Usually when there's a mistake it will eventually be corrected automatically by HMRC but that can leave you with an unexpected extra tax bill (or a refund, which is usually less of a problem), but those corrections happen months after the end of the tax year.

Also if you pass a certain income threshold you're required to fill in your own tax return anyway.

> HMRC does sometimes make mistakes with people's tax codes, resulting in > the wrong amount being taken off your pay, and your employer doesn't > (and can't) correct those mistakes for you

That is correct. However, HMRC sends you a letter with every tax code change, which is calculated and posted automatically; I've once received two separate letters, with two different tax codes, sent literally one day apart from each other. :D

And if you feel that you tax code is incorrect it is very easy to check your online HNRC account and correct any information they might have.

Can attest to being bitten by an unexpected tax bill, after a previous employer years before had the wrong tax info for me.

Anyone earning £100,000+ in the UK does have to file a return, however most of the calculations are done for you. Your employer handles all deductions from your payslip and gives you a summary form of all tax paid, which you then report in your tax return with additional details that might affect your taxes such as capital gains, additional pension contributions or investment and rental income.