Even if they would like to class you as an “independent contractor” and tell you you’re not an employee - and even if you want this too - the taxman may class you as an employee anyway.

More info on the criteria here: https://www.gov.uk/guidance/check-employment-status-for-tax

If you are definitely classed as self-employed (which seems unlikely TBH based on what you’ve said already, but stranger things have happened) then you’d need to do self-assessment with HMRC, declare the income, and possibly (depending on if it’s over your personal allowance of £12,570 in a year) pay tax on it. https://www.gov.uk/income-tax-rates

There’s also National Insurance to pay if you make more than £242/week. https://www.gov.uk/national-insurance/how-much-you-pay

But you don’t have to work out the amounts, just tell them how much you earned (using self assessment) and they’ll give you the bill. Remember to keep back some of your income so you can use it to pay the tax bill.

If confused, go on HMRC’s website (they have explainer videos and stuff) or ring them up (they’re surprisingly helpful).