@MostlyHarmless This is a peculiarly American problem driven by two special-interest groups:

• 'Small government' conservatives want paying tax to be difficult so that people hate it.
• Companies that produce software for completing tax returns want it to be difficult so that they can sell software to do it.

In the UK, most people never complete a tax return. They pay through Pay As You Earn (PAYE) where the tax is deducted each month as you pay. If they over or underpay one year, the PAYE rate is adjusted the next year to compensate (unless it's a huge amount). If you do have to complete a tax return, the employment bit is usually easy: it should be prepopulated, but if not then you just need to copy a couple of numbers from a standard form that every employer is required to give you. It's only difficult if you have a load of investments and other income, and even then you just fill in a web form with the information and they tell you how much you owe. If it's a smallish amount (a few thousand or less) then they adjust your PAYE rate next year to collect it. Otherwise you have to pay it, but they tell you exactly how much.