I've been doing UK self assessment tax returns every year for over 15 years.
It really does just take 10-15 mins for people that have one job (as an employee) and typical investments - savings accounts, shares, etc.
The income numbers are already there and if I want to check it's easy: my employer gives me a form with the same numbers in the same numbered boxes.
I just need to specify how much income I had from bank interest.
The tax witholding system usually works as well - the main exception being straight after starting work for the first time or changing jobs, when you can have a temporary code. In these cases I just called HMRC and told them what was going on.
The employer gives my pay numbers to HMRC and HMRC give my employer a tax code that determines how much to withhold each month.
The income numbers are already there and if I want to check it's easy: my employer gives me a form with the same numbers in the same numbered boxes. I just need to specify how much income I had from bank interest.
The tax witholding system usually works as well - the main exception being straight after starting work for the first time or changing jobs, when you can have a temporary code. In these cases I just called HMRC and told them what was going on. The employer gives my pay numbers to HMRC and HMRC give my employer a tax code that determines how much to withhold each month.