Problem joining MayDay breakdown.

I just tried to join MayDay breakdown service online and encountered a problem as my vehicle has a private numberplate which it will not accept.  I am assuming it trying to look for an 'age related'  numberplate?  Has anyone else encountered this problem before?

I have real reservations about phoning Green Flag and handing over my credit card details, as many of these call centres are not located in the UK and having had a card compromised in the past, I am wary about giving those details out over the phone.