...Cash in hand,,,,,, and HMRC will take a very dim view on it. As for asking for a cash in hand job on a public forum ...
Cash in hand and being paid in cash can quite easily mean different things to different people.
In principle there are two key aspects at work, these are employment law and tax law.

As far as tax law and HMRC is concerned it means only one thing.....you get paid cash for work done.

There's nothing wrong with being paid cash, provided it's not for the purpose of avoiding tax.

If someone pays you less than £109 a week and you declare that you have no other job and no other taxable income (even a pension) then neither you nor they need to do anything. HMRC don't require notification.