IIRC these costs depend very much on the country in which one holds EC membership. Cancellation online at ba.com also tends to be less costly (I think free under at least certain circumstances) whereas over the phone there is almost always a charge (unless a GCH, and even this may depend again on country of registration).

As for the general lack of a 24-hour (or even somewhat shorter) policy to deal with obviously unexpected fumbles, I find it very Ryanair-esque that BA chooses to take the hard line that no errors can be corrected, in contrast to so many airlines who encourage use of their websites by publishing a "risk-free" booking policy, which usually means you can do anything to change/cancel in the first 24 hours, regardless of circumstances. Add to this that BA.com doesn't even allow you to place a reservation on hold (as virtually every carrier does that does NOT have a 24-hour cancellation policy) and one has a rather hard time concluding anything other than that BA have missed the plot on how to ensure that the occasional mishap doesn't leave some stains in rather too many customers' experiences of online booking. I wouldn't be surprised of this behavior from Ryanair, but I don't expect it of British Airways. Do forgive me for holding a higher opinion of them than that...

Thankfully, has never happened to me on a BA booking, but has once or maybe even twice in the last 10-15 years with some other carriers.

Lastly, when confronted with a customer-hostile policy from another country's equivalent of Ryanair - and I was making a correction literally minutes after an initially botched booking, I called my credit card issuer, explained what happened, and they canceled the charge - in spite of the airline not being willing to deal with it. So that is sometimes an option, albeit a slightly extreme one.