Is it illegal for you to work cash in hand in the UK?

Answer 1

No, but you would need either of the following two scenarios to be true for it to be legal:

Either: you would be registered as Self Employed and file a tax return which detailed how much money you were paid during the previous tax year. You would then pay the tax for the whle year in one lump sum during the next tax year. You would also pay your National Insurance quarterly. Not declaring the income that was in the form of "cash in hand" would be an offence.

Or: your employer deducted all of the tax and National Insurance contributions and paid the government on your behalf, then gave you the cash that was left as your wages. You should get a payslip as proof that this had been done.