The tax you pay will be the same which ever job you say is your main job. HMRC are interested and will tax you on your total income, regardless how many jobs you have
It is good practice to tell HMRC about both your jobs. If you do not tell them yourself. They will find out when your second employer makes their annual return of employers earnings and tax, so you would be chased for tax due a few years later.
More to the point, your permanent employer may put you on emergency coding until they get a tax code or letter of exemption from HMRC so you will be paying more tax until the problem is resolved.
Reforms response to Rachel Reeves’ heckler.
