ok - on a slightly more constructive note, and fresh from my victory against helmet compulsion on the Martlets ride, I'd suggest the following
Ask who has insisted on helmets. It might be the firm, it might be the charity, it might be the broker or it might be the underwriter
If it's the firm I'd suggest a polite letter saying that compulsory helmet use dissuades people from riding bikes, and, with regret you can't take part in the ride
If it's the charity I'd suggest a polite letter saying that compulsory helmet use is off-putting and suggest that they would get more riders if they dropped the requirement. In the mean time, you're sorry, but cycling with a helmet is no fun, and you'd rather be at work
If it's the broker then write to the employer, the charity, and the broker saying that other charity rides are organised without a compulsory helmet rule, which, in your opinion, will diminish the appeal of the ride, and that you would suggest that the broker looks at the way the ride is managed, including the risk assessments, particularly of the route, the safety and highway code advice given to riders, and any checks made on the bikes along the lines of the Dr. Bike checks, and make a proposal to the underwriter with that information to hand
If it's the underwriter then write to the broker, copying your letter to the charity and the firm saying that other insurers don't have a problem, and you would be happy to offer names (Towergate spring to mind and I can find you some others)
You could also write to the firm and suggest that if they're seriously going to back cycling then the sensible thing to do is to set up a CTC affiliate, sell the CTC third party insurance to employees (it's well worth having), and run the event under CTC rules, which, the CTC having had a fair amount of experience of this kind of thing, won't require helmets. That takes time, though, and is probably one for next year.
Think of this less as a personal choice and more as an opportunity to strike a blow for freedom of choice for all