You can try the CAB and what they will probably tell you is that he is changing your terms and conditions of employment, which he can do by giving you at least 2 weeks notice. They may also tell you that you could resign and claim constructive dismissal at an Employment Tribunal. You might win and get financial compensation of a couple of �K. You wouldn't get the job back on the original terms.
There isn't an automatic link between poor HR policies and the IR (I think we mean HMRC actually) 'trawling through his business accounts' - it simply won't happen.
There is no obligation to pay overtime rates to anyone.
Holiday pay should be provided to at least the statutory minimum levels.
There are no fines for breaching employment laws - they are breach of contract situations.
Keep up the day-job, Tamborine, you have a heck of a lot to learn before you can provide quality information on this subject that people can actually use.