[lozenge]

It's achieved in three ways

1) far less deductions/carve outs in tax law, or ones that people can use without contacting the tax service. Eg pensions and charity contributions deducted by employer, so tax is calculated. Charity contributions paperwork for one off donations dealt with by the charity (Gift Aid scheme)

2) "tax code" five digits representing your tax status, that the tax office will give your company to update calculations when your status changes. Possibly starting from a phone call from you. Eg using marriage tax laws.

3) tax summary P60/P45 provided by your last employer(required by law) that you can give your next employer so they calculate correctly

Sounds complicated, but it's all geared towards moving the burden to employers who just pay for payroll software, for everybody else it is very simple.