Right, let's try and get this sorted!
Firstly, the right to be accompanied:
You are being invited to an investigation. There is a (minor and techy) argument about the right to be accompanied to pure investigation, but let's run with it for the moment.
You have the right to be accompanied by a colleague or a trade union official. You don't have any right to take a friend or a solicitor. However, you don't need to be in a union to take a union rep, so if your cousin or next door neighbour happens to be one, lucky you!
Next, steps pre meeting.
I would suggest that you send your employer an email indicating that you are very concerned about the letter you have received as it is so vague as to make it impossible for you to properly understand the allegations and prepare. If you think that there are particular documents you need (like calendar to respond to their request), then say that too.
At the meeting:
This is an investigation, so you don't necessarily get witness statements in advance. A genuine investigation would interview everyone involved and take their statements. If it progressed to an actual hearing then that's the stage at which you would get the statements (with the letter inviting you to the hearing).
So, at the hearing, your employer should be explaining the allegations in more detail and you should answer as best you can. If it's a vague allegation - e.g. that you use hostile body language - say something like "I don't believe that that is the case, but it's really hard for me to answer something so vague. Please could you explain what I am alleged to have done, to who and when, then I will try and give a fuller answer".
Unlike others, I wouldn't advise ever saying that you don't want to answer until you've had legal advice. It comes across a bit like the guy stopped by a policeman who suddenly goes "I want to talk to my lawyer." Instead, I would focus on practical reasons why you can't answer, or aren't happy to do so.
I would suggest that you repeatedly ask who has made the allegations (if you don't get a satisfactory answer). If they won't answer, or it's one of this group, state your concerns abou them as a group.
If you can get a colleague to go along with you, so much the better as he/she can take notes.
On the expenses and requests for dates:
I suggest that you email them and ask them whether this is part of the investigation or a separate issue. Then explain your difficulties with answering. If you want, throw in an aside about the witchhunt (although try to use very measured language).
Finally, claims:
Just so you're clear, the claim if they dismiss you is unfair dismissal. Constructive dismissal is a claim you make if you feel that you've been treated so badly you are entitled to resign and treat yourself as dismissed. It's a hard claim and I only advise that route if you are being badly treated but with no suggestion you'll be fired (e.g. sexual harassment or bullying). If you think you might be dismissed, waiting it out and claiming unfair dismissal is an easier and more straightforward route.
Hope that helps.