What to do if your boss doesn't pay your tax and National Insurance.

If you are working for somebody and you aren't getting paid 'on the books' or getting paid 'under the table' as they say in the US, chances are you're either A) getting paid less than the minimum wage, or B) getting paid without your boss paying your taxes and National Insurance contributions. If either of these apply you're not alone, despite the massive lack of advice available to you, there are things you can do to help yourself.
The first and most obvious option is to leave your job, and don't worry if you feel some kind of loyalty to your employer because they ARE screwing you over, if they 'can't afford to pay it' or some other lame excuse in my opinion they should never have started in business in the first place. Maybe business is slower than they anticipated or things don't seem as rosy as your employer promised, this is not your fault, nor is it your problem.
The second option is more complicated, and may still involve leaving your job (when your employer gets sent to prison for fraud) gather whatever evidence you can about your predicament and contact your local Citizens Advice office, you will need evidence of this because if it gets as far as your employer finding themselves in court with a fraud charge against them, they will be looking after number 1 plain and simple, they won't care about you as they see themselves as having much more to lose than you do.
Good luck, and remember, you are not alone and carrying on in your situation will definitely have a devastating effect on your life sooner or later.