Most of the big ones can be done without a self assessment. If you make a pension contribution for example, you can call hmrc and tell them and they'll adjust your tax code
Not at all. The list of things you can claim for is very limited, and the majority of them are automatically applied. Having to actually contact HMRC is a rarity. If you are salaried employee with no extra cash coming in, you don't need to do anything.
Another example of how the system works - if you are working two jobs, both as a regular employee, HRMC will instruct your employer as to what your tax deduction should be for that paycheck to make sure that things are balanced. For <some large number>% of people, this will be correct, and if it's not, it will rectify itself over 2 or 3 pay periods. If that's _still_ not enough, a phone call to HMRC (usually taking less than 10 minuts for the two times in a decade I've had to do it) will resolve the issue in your next check.