Universal Credit get their info of your income from HMRC. If you are paid night out money and doesn’t involve HMRC, UC wont know.
Unfortunately that also means if you are given a tax refund by HMRC, UC count it as income and you get a deduction in what you qualify for due to that.
If you go the Universal Credit route, look up their “Help to Save” scheme. It is generally kept quiet but if you can find the money each month, you get a nice bonus after each 2 year period. Martin Lewis done a segment on this a while back. You have to be on UC to start the account but if you finish UC for example a couple of months on, you can keep the Help to Save account running long after you finished UC and still receive the bonus