I think you need to be more realistic than many of the sympathetic answers you have here.
No, I don't think you will, nor should go to prison for this.
But I don't think it's anything like as clearshot as you will win a fight against them.
But what you've written here, if I was investigating, would definitely raise alarm bells.
I've been in a situation with someone committing fraud, not to the extent you have, and everything had an answer which seemed reasonable for each individual incident. Just the number of "accidents" was a little bit hard to believe, and I think you'll be in that situation.
"I was distracted", "I meant to let them know", "It was an accident", "of course I'll pay what I can back", "I've had a terrible time recently..." and it sounded a terrible misfortune... until an email was found which made it clear that it was deliberate and further investigation showed that it had gone back years.
The two things that I noticed from your Op is:
I disputed some which were left on an agree to disagrees ; lots of people think the 9% was bad. There was more. How much more? If you're talking about another £100, then that's not that bad. If you're talking about another £900 then that's worse if it's more than that, then I think you are looking in bad waters.
And on what grounds are you disagreeing? That you did the miles, or that you didn't claim them? Because you haven't left it as they agree you didn't fraudulently claim: you've left it as they still think you did; you say you didn't. So their investigation will be taking those into consideration, and looking at those.
The other thing is:
It’s been a tough time recently, lost my mum and then I got Pneumonia and ended up in intensive care for six weeks
That's only really going to wash as an excuse if all, or pretty much all, the events were after this. If that is the case, then you'll probably have a reasonable case for asking them to take this into consideration; but if you are still on O2, so it sounds like most were before, then it looks like an excuse.
Yes, I would phone ACAS, and maybe a solicitor. But you need to be honest with them.
Are they going to find, if they investigate, more claims dating back years?
Why do some of the claims get automatically cancelled, and others don't? Is it something you should have done?
And how much total are they saying you claimed fraudulently?
What's your defence for the ones you disagree about?
Are there likely to be others in the same situation?
Have you ever had a similar accusation before, or been caught out on expenses?
If you're not honest with them, then they can't give you the advice you need and then you may be barking up the wrong tree, which is unlikely to help your case.