Well you've got me confused, Joko.
If you're genuinely 'freelance' you should be self-employed and paying any relevant tax directly to HMRC. There shouldn't be any deduction of tax by an 'employer' since you haven't got one. (You should be paid the full amount due by the person you've contracted your services to and then sort out the tax, for yourself, with HMRC).
If you're actually doing a second employed job then, assuming that your total pay (across both jobs) isn't high enough to take you into a higher tax bracket (and assuming that you've earnt enough from your first job to use up your personal allowance) you should be paying tax at 20% on everything you earn. So you should have paid £30 in tax. (You should also pay National Insurance on your second job as well but, with only £150, that will be almost negligible).
Chris