Unfortunately there is no county field, we decided against this back in the early days as it was really only applicable to UK based companies, and back then we planned to offer our service in many other countries, so we tried to keep it as streamline as possible. We’re kind of stuck with that decision now as the system has expanded so much that it would be a huge undertaking to retrofit this into the code.
I still have some reservations about using county, as I believe Royal Mail don’t officially recognise the county as a critical part of a UK address (The following quote was referenced from Wikipedia):
According to the Royal Mail, the former postal county data no longer forms part of postal addresses. It was removed from the Postcode Address File database in 2000 and does not form part of its code of practice for changing addresses.