You can certainly give the details in the form of a letter (Case law, Jones V DPP) which may inconvenience them somewhat (and may see them withhold the offer of a course or fixed penalty as applicable).
Otherwise, as per reply on Pepipoo, if mail is the only option they offer, then it is the only option.
Not sure why you don't want to log on, given you'll only be providing details they already have..... But calibration (and the resulting certificate) isn't required to convict you, so not sure why you want to see them anyway?