I think a good place to start is what
quimcunx said - "what do you actually want to have happen?".
From my reading of the situation, it comes down to two main possibilities, if we discount any kind of personal/revenge motive.
1. Finding out if all the money that's been donated in the past went to charity.
2. Making sure that future charitable donations DO go to charity.
As far as 2. goes, I think kebabking nailed it - IF she has been taking the money for herself, it's pretty serious. She's been using the names of charities to collect money from fellow employees, so she'd be screwing over her co-workers, her employers, and the charities. So from that point of view, your employers should certainly have a say in what happens, but I think you need to be pretty sure they'll handle it sensitively. If you're not up for a witch hunt, it'd be fairly shit if the company decided to make it into one.
I'd be very tempted to discuss it with a reasonably senior manager within your office: that is, after all, their job - to manage stuff...
Collecting for charity and then keeping the money is Fraud, a serious criminal offence, and if she has used your departmental facilities - work email, time, premises etc.. to attempt to legitimise her activities, then that would be Gross Misconduct. For a civil servant that would be immediate dismissal...
I think you'd be unlikely to get much from the charities involved - what one person chooses to donate, whether on other people's behalf or not, is no business of anyone else, and unless you're in a position to represent your department officially, they would be very unlikely to tell you what donations have been received in your offices name.
Personally I don't understand the reluctance get involved - assuming it's genuine belief that money given to charity has been stolen then there is no moral problem with asking the appropriate authorities to investigate. If she's innocent then the investigation will last 10 minutes and she'll be exonerated, and your department will put in place rule regarding collections to ensure the same thing doesn't happen again - I'm surprised they don't, we do - if on the other hand she's been lifting several thousand a year from your colleagues pockets, then why would anyone object when she gets sacked and potentially prosecuted...?
If that all seems a bit heavy, then I think
PippinTook's solution covers the bases nicely - focus on what happens from here on in. Get your employer to implement a policy that states than any collections on company premises/time are properly receipted and acknowledged.
I wouldn't do anything that points a finger at her without absolute proof. Cakes? Not worth making an issue. Charity dress? Not worth making it a jobsworth issue.
Money to charity? Definitely jobsworth issue but you have no proof and do you really want her to lose her job?
It's a case of damage limitation now to be honest. Get a policy going on charitable collections. Tell management you're all sick of all the collections and then work on the policy yourselves.
Make sure that there is an agreed number of charities and collections per year. And that all moneys must be accounted for and sent via company cheque to the charity. That way she never gets to touch the money...
Also...ensure charities give your company a receipt.
Going after her for a charity dress and cake sounds petty tbh.
Shut down the whole charity collection stuff by regularising it and controlling it...from management down. That will sort her out...fast.
If it means she "gets away" with the previous (potential) thefts, then that might be a reasonable price to pay for not being seen to be launching some kind of vendetta against her. It might be that if you go for the
PippinTook Scenario someone else will go "hmm, so what's all this about?" and deal with matters themselves, but that may just be a risk you have to take.
If I were a manager in a company where someone was very actively collecting for charity, no matter if there were suspicions or not, I think I'd like to have some degree of oversight into what was going on - and maybe, as someone else suggested, I might even want to do a bit of donation-matching.
ETA: I should declare an interest. I get pretty fucked off with endless demands for money, especially when accompanied by heavy doses of moral blackmail, in the name of "charidee". So, the chances are I'd probably have demurred a long time ago if PudseyWoman had been a colleague of mine: "no thanks, I do my own donating privately."