UKCasual

It's really targeted at people who want to stay "off the books" or not pay any taxes, so is what it is. If you're getting paid cash up front then there's not too much issue for you.

Technically, you have an employee - employer contact by law automatically, and they legally need to provide you with a letter stating certain terms within x days of the start date. I can't remember exact details.

They should also be putting you through payroll, deducting taxes (if needed), and paying employers NI. Someone else mentioned the need to pay you minimum wage by law

These are all additional costs to them so if you raise it as an issue with them, I'd say they will cut you out and block you. You're unlikely to get any authority to really care, especially if they keep a low profile for this purpose.

It might be a good idea to personally tell the tax authority about your earnings (ignoring the employer) and keep track of it. If in a tax year (April to March) you earn less then £5k you won't pay any NI and less than £12.5k you won't pay any income tax. It's quite straightforward to do and if you're not paying tax then no real cost. However, you could also easily get away with doing nothing, especially if they pay you cash and there's no real record. Feel free to message me to ask any more questions about tax.