It can be more tax efficient to file tax paperwork, but I don't want to do paperwork, so I just had an umbrella employ me when somebody insisted on hiring me as a "contractor" and the umbrella handled the paperwork for which of course they keep a fee. As far as the government is concerned I just had two PAYE employers, the umbrella and my "real" job.

Having multiple employers also does not require filing taxes. One of the employers gets given a zero tax code and they tax all your pay at full rate, the other one gets a normal tax code which reflects your personal allowance and other considerations. My tax code was oscillating all over the place - but that's not a problem it's all automated.

If you love paperwork you can choose to do all the paperwork. Or if you love money and don't hate paperwork maybe you can save a few hundred quid by filing and I hope it makes you happy. I hate paperwork, and I have plenty of money. So, no, despite earning a lot of money and having worked as a contractor I preferred to stick with PAYE.