As others have stated, you can't legally do any 'cash in hand' jobs (unless, of course, you tell both the benefits agencies and the tax man about it).
Bar work used to commonly offer 'cash in hand' but most publicans won't take the risk these days. The same applies to market stallholders. A friend of mine used to pay the staff on his burger van cash in hand but he got worried about possible legal action and switched to putting everything through the books.
You won't find any particular types of regular jobs where all (or most) employers offer 'cash in hand'. Many types of casual work (e.g. traffic surveys, directory deliveries, etc) are offered on a self-employed basis where it's left to the employee's honesty to declare the earnings but payment is made by cheque (which will, of course, show up in the worker's bank statements if the benefits agencies ask to see them).
Some people try to get round the system by going genuinely self-employed (and 'forgetting' to tell the relevant bodies). These include 'car-booters' (other than those genuinely selling just their own personal possessions) and eBay traders (ditto). It's still illegal but those who just do it on a small scale often get away with it.
Chris