Some thoughts
unions don’t have to be in your workplace to be part of one. You just have to find the one suitable for your role/profession.
but a union won’t help you now, it’s far too late for a pre-existing issue of this complexity
ACAS will provide guidance on process but absolutely won’t be there to support you. It is not their role to do that. If you go down an unfair/constructive dismissal path ACAS will act as mediators (and you have to do this before tribunal). And remember the 3 months less one day rule, so stuff that happened over a year ago may be ruled out (although it would give context).
not sure CAB will be of help but try them. They may not be specialists in employment law. An employment lawyer is what you need (a friend unless with specific knowledge can only edit based on the readability of your grievance not the content and law / policy aspect).
grievences should start with the solution you are seeking - what do you want out of this process? What do you want the employer to do?
your grievance letter should clearly state the facts but it should be considered. The longest draft grievance that I have read (as an accredited rep) was 120 pages in length. As the rep I gave up after the first 10 pages, as the employer they would haven’t got so far.
you need to stick to facts and try not to get emotional. When there has been an impact you need to be clear in what that impact has been. Being specific against policies and laws is useful.
you need to get a lawyer to advise you and a good one (I know some duff ones out there who will take anything on and have no hope of success). They can help frame your grievance in terms of the law and policies (and if the employer has followed the policies which generally they don’t) and give some indication of your success if you have to go to tribunal.
as an aside and what I say to my members - grievances are awful for all involved and beware, there are always unintended consequences- you will also be put under a spotlight so ensure that you have clean hands and have also behaved appropriately